Nitride semiconductor device comprising bonded substrate and fabrication method of the same

ABSTRACT

A substrate  1  for growing nitride semiconductor has a first and second face and has a thermal expansion coefficient that is larger than that of the nitride semiconductor. At least n-type nitride semiconductor layers  3  to  5,  an active layer  6  and p-type nitride semiconductor layers  7  to  8  are laminated to form a stack of nitride semiconductor on the first face of the substrate  1.  A first bonding layer including more than one metal layer is formed on the p-type nitride semiconductor layer  8.  A supporting substrate having a first and second face has a thermal expansion coefficient that is larger than that of the nitride semiconductor and is equal or smaller than that of the substrate  1  for growing nitride semiconductor. A second bonding layer including more than one metal layer is formed on the first face of the supporting substrate. The first bonding layer  9  and the second bonding layer  11  are faced with each other and, then, pressed with heat to bond together. After that, the substrate  1  for growing nitride semiconductor is removed from the stack of nitride semiconductor so that a nitride semiconductor device is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a fabrication method of a nitride semiconductordevice comprising a nitride semiconductor (In_(x)Al_(y)Ga_(1-x-y)N, 0≦x,0≦y, x+y≦1), such as a light emitting diode (LED), a laser diode (LD);light receiving devices such as a solar cell, a photosensor; orelectronic devices such as a transistor, a power device; and the like.

2. Description of the Related Art

Owing to the stability in the high temperature ammonia atmosphere in itsepitaxial growth process, it has been proved that sapphire is adesirable substrate for growing a light emitting device of a highefficiency made of nitride semiconductor. Practically, a nitridesemiconductor device grown on a sapphire substrate is employed for ahigh brightness blue emitting LED, a pure green LED, and LD (laserdiode) and it is applied for full color displays, signal displayapparatus, image scanners, light sources such as a light source foroptical disks and for media such as DVD that stores a large quantity ofinformation, or printing apparatus. Further, applications to electronicdevices such as field effect transistors (FET) are expected.

Although a nitride semiconductor is a promising semiconductor material,its bulk crystal production is difficult. For that, presently,hetero-epitaxial techniques for growing GaN by metal organic chemicalvapor deposition (MOCVD) on a hetero-substrate of such as sapphire, SiCand the like are widely used. Especially, in the case of using asapphire substrate, a method involving forming AlGaN as a buffer layerat a temperature as low as about 600° C. on a sapphire substrate andthen growing a nitride semiconductor layer thereon is employed.Crystallinity of the nitride semiconductor layer is improved by such amethod. Japanese Patent Application Laid-Open No. 2002-154900 disclosesa structure comprising a Ga_(x)Al_(1-x)N as a buffer layer and a crystalof a gallium nitride-based compound semiconductor grown thereon.

SUMMARY OF THE INVENTION

However, sapphire is an insulator with a low thermal conductivity and itlimits the structure of a device. For example, in the case of aconductor substrate of such as GaAs and GaP, electric contact parts(electrode), one on the top face of a semiconductor apparatus andanother in the bottom face, can be formed, whereas in the case of alight emitting device grown on sapphire, two electric contact partsshould be formed on a top face (on a single face). Therefore, if aninsulating substrate of such as sapphire is used, the effective lightemitting surface area for the same substrate surface area is narrowed ascompared with that of a conductor substrate. Further, in the case ofusing an insulating substrate, the number of devices (chips) that can beobtained from a wafer with the same Φ is low.

Further, there are face-up type and face-down type devices as nitridesemiconductor devices using an insulating substrate of such as sapphireand since both electrodes exist in a single face, the current density isincreased locally and heat is generated in devices (chips). Also, sincewires are required respectively for both p and n electrodes in wirebonding process for the electrodes, the chip size becomes large toresult in decrease of yield of the chips. Furthermore, sapphire has highhardness and a hexagonal crystal structure. Therefore, in the case ofusing sapphire as a growing substrate, it is required to subject thesapphire substrate to chip separation by scribing, and the fabricationsteps are increased as compared with those in the case of othersubstrates.

On the other hand, in the case of a nitride semiconductor device usingSiC as a substrate, since SiC is conductive, electrodes of a device canbe formed in both top and bottom faces. However, in the case a nitridesemiconductor device is grown on a SiC substrate, no electrode can bedirectly formed in the side of the nitride semiconductor layer where thelayer contacts with the SiC substrate. That is, although one electrodecan be formed directly in the nitride semiconductor while contactingwith the semiconductor, the other electrode has to be formed in the rearface of the SiC substrate. This means that electric current flowsthrough the SiC substrate. However, the conductivity and the thermalconductivity of SiC are not sufficient.

For that, in consideration of the foregoing problems, the invention aimsto provide a nitride semiconductor device having an opposed electrodestructure in which both electrodes are on the opposite to each other insuch a manner that a p-electrode is formed in one main face of a stackof nitride semiconductors and an n-electrode is formed in the other mainface and its fabrication method.

Further, recently, LED emitting light with short wavelength in an UVregion or the like is made practically applicable. FIG. 18 shows aschematic illustration showing one example of the structure of a nitridesemiconductor device emitting light in an UV region. A GaN buffer layer52, an n-type GaN contact layer 53, an n-type AlGaN clad layer 54, anInGaN active layer 55, a p-type AlGaN clad layer 56, and a p-type GaNcontact layer 57 are laminated on a sapphire substrate 51 and ap-electrode 58 is formed on the p-type GaN contact layer 57 and ann-electrode 59 is formed on the n-type GaN contact layer 53 exposed byetching. The luminescent wavelength can be changed by changing the Incomposition ratio of the active layer and the wavelength of theluminescence wavelength is shortened by lowering the In compositionratio.

However, if it is tried to shorten the luminescent wavelength, forexample, to shorter than 365 nm, which is a band gap of GaN, it becomesdifficult to utilize a quantum well structure of InGaN, which isconventionally used as an active layer to result in a problem thatsufficient luminescence output cannot be obtained. Further, there isanother problem that the light outputting efficiency is considerablydecreased owing to absorption by a material having a band gap near tothe luminescent wavelength.

Therefore, another aim of the invention is to solve the above-mentionedproblems, suppress heat generation of a device by making the electriccurrent distribution even, and provide a nitride semiconductor devicehaving a high luminescence output even in the UV region and itsfabrication method.

The fabrication method of the first embodiment of a nitridesemiconductor device comprises:

(a) forming a stack of nitride semiconductor by growing at least one ormore n-type nitride semiconductor layers, an active layer having aquantum well structure including at least a well layer ofAl_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0≦b≦1, a+b≦1) and a barrier layer ofAl_(c)In_(d)Ga_(1-c-d)N, (0≦c≦1, 0≦d≦1, c+d≦1), and one or more p-typenitride semiconductor layers on one main face of a substrate for growingnitride semiconductor that has two mutually opposed main faces and has athermal expansion coefficient higher than those of said n-type andp-type nitride semiconductor layers;(b) forming a first bonding layer including one or more metal layers onsaid p-type nitride semiconductor layers;(c) forming a second bonding layer including one or more metal layers inone main face of a supporting substrate having two mutually opposed mainfaces and having a thermal expansion coefficient higher than those ofsaid n-type and p-type nitride semiconductor layers and equal to orsmaller than that of said substrate for growing nitride semiconductor;(d) setting said first bonding layer and said second bonding layer faceto face each other and pressing said stack of nitride semiconductor andsaid supporting substrate with heat to bond together; and(e) removing said substrate for growing nitride semiconductor from saidstack of the nitride semiconductor.

The substrate is preferably conductive, more preferably made of a metalor a metal composite. Since a metal or a metal composite is not onlyhighly conductive but also excellent in thermal conductivity, it canimprove the heat releasing property of a nitride semiconductor device.Incidentally, a “conductive” substrate in this invention includesconductive metals and semiconductors as well.

According to the fabrication method of the first embodiment of theinvention, the above-mentioned nitride semiconductor layers includingthe active layer are grown on the substrate for growing nitridesemiconductor, the stack of nitride semiconductor for bonding and thesubstrate are bound each other and then the substrate for growingnitride semiconductor is removed, so that a p-type electrode and ann-type electrode can be formed on the p-type nitride semiconductorlayers and on the exposed n-type nitride semiconductor layers,respectively. Consequently, the p-electrode and the n-electrode can bearranged face to face each other so that the electric currentdistribution can be made even and heat generation of the device can besuppressed. As a result, a nitride semiconductor device with a highluminescence output in the UV region can be provided.

Inventors have found a remarkable relation of thermal expansioncoefficients of a supporting substrate and a substrate for growingnitride semiconductor in order to prevent chipping and cracking of thenitride semiconductor layers. That is, the chipping and cracking of thenitride semiconductor layers can be remarkably suppressed and a nitridesemiconductor device with high reliability in production yield can beobtained by making the thermal expansion coefficient of the supportingsubstrate equal to or smaller than that of the substrate for growingnitride semiconductor. In other words, three thermal expansioncoefficients A, B, and C of the substrate for growing nitridesemiconductor, the nitride semiconductor layers, and the supportingsubstrate, respectively, are controlled to be as A=C>B or A>C>B.

The mechanism of the prevention of the chipping and cracking of thenitride semiconductor layers depending on the relation of the thermalexpansion coefficients is supposed to be as follows. It will bedescribed simply with reference to FIG. 2 to FIG. 4. As a simpleexample, the case that a GaN layer 23 is grown on a sapphire substrate22 as a substrate for growing nitride semiconductor and a supportingsubstrate 24 is bound to the GaN layer 23 will be exemplified. Thethermal expansion coefficient A of sapphire is about 7.5×10⁻⁶ K⁻¹ andthe thermal expansion coefficient B of GaN is about 3.17×10⁻⁶ K⁻¹ in thec-axial direction and about 5.59×10⁻⁶ K⁻¹ in the a-axial direction.Since the difference of the thermal expansion coefficient in the planedirection of the mutually bonding interface is concerned in thisinvention, in the case of GaN grown in the c-axial direction on thec-plane of the sapphire substrate, the difference of the thermalexpansion coefficient of GaN in the a-axial direction should beinvestigated. Supporting substrates having a variety of thermalexpansion coefficients C will be formed thereon.

First, the warping of a wafer in the case the relation of the thermalexpansion coefficients is “C>A>B” is as shown in FIGS. 2A to 2B. Whenthe GaN layer 23 is formed on the c-plane of the sapphire substrate 22,the warping occurs with the GaN layer 23 side being projected as shownin FIG. 2A. Next, when the supporting substrate 24 having a thermalexpansion coefficient C larger than the thermal expansion coefficient Aof the sapphire substrate is bound to the GaN layer 23, the warpingdirection is reversed as shown in FIG. 2B. At this time, due to largestrain given to the GaN layer 23, the GaN layer is subject to problemssuch as craking or peeling.

Second, the warping of a wafer in the case the relation of the thermalexpansion coefficients is “A>B>C” is shown in FIGS. 3A to 3D. In a firststep, the GaN layer 23 is formed on the c-plane of the sapphiresubstrate 22, the warping occurs with the GaN layer 23 side beingprojected as shown in FIG. 3A. When the supporting substrate 24 having athermal expansion coefficient C smaller than the thermal expansioncoefficient B of the GaN layer is bound to the GaN layer 23, the warpingdirection is not changed as shown in FIG. 3B. Next, after the wafer isturned upside down as shown in FIG. 3C, the sapphire substrate 22 isremoved as shown in FIG. 3D. In this case, the warping direction is notchanged before and after removing the sapphire substrate 22. The warpingshape is as the sapphire substrate 22 side being recessed. Therefore, itis difficult to remove the sapphire without causing cracking and peelingof the GaN layer 23. For example, if it is tried to remove the sapphiresubstrate 22 by polishing, the polishing is promoted only in theperipheral part and even polishing is difficult. If it is tried toremove the sapphire substrate 22 by laser beam, the sapphire substrate22 is hardly floated up from the GaN layer. Accordingly, the removal ofthe sapphire substrate 22 is difficult and if it is tried to forciblyremove the sapphire substrate 22, cracking and peeling of the GaN layer23 tend to take place.

Third, the warping of the wafer in the case the relation of the thermalexpansion coefficients is “A≦C>B” is as shown in FIG. 4. When the GaNlayer 23 is formed on the c-plane of the sapphire substrate 22, thewarping shape is as the GaN layer 23 side being projected as shown inFIG. 4A. When the supporting substrate 24 having a thermal expansioncoefficient C equal to or slightly smaller than the thermal expansioncoefficient A of the sapphire substrate is bound to the GaN layer 23,the warping is reduced as shown in FIG. 4B. Next, after the wafer isturned upside down as shown in FIG. 4C, the sapphire substrate 22 isremoved as shown in FIG. 4D. In this case, when the sapphire substrate22 is removed, the warping shape is changed to a shape with the sapphiresubstrate 22 side being projected. Accordingly, in the case of removingthe sapphire substrate 22 by polishing, polishing is easy to be carriedout evenly. Also, in the case of removing the sapphire substrate 22 bylaser beam, the sapphire substrate 22 is easy to be floated up from theGaN layer. Consequently, at the time of removing the sapphire substrate22, occurrence of cracking and peeling in the GaN layer can besuppressed.

As described above, by controlling three thermal expansion coefficientsA, B, and C of the substrate for growing semiconductor, the nitridesemiconductor layers, and the supporting substrate, respectively, to be“A≦C>B”, the chipping and cracking in the nitride semiconductor layerscan be remarkably decreased and a nitride semiconductor device can beobtained at a high production yield. Further, keeping the thermalexpansion coefficients in the above-mentioned relation is advantageousin a point that the process after removal of the sapphire substrate 22is made easy. That is, as shown in FIG. 4D, since the warping is finallyin a state that the nitride semiconductor layer 23 side is projected,the nitride semiconductor layer 23 is easy to be flat by vacuumadsorption adsorbing the supporting substrate 24 side. Accordingly, itis made possible to evenly polish the nitride semiconductor layer 23 andto evenly form a resist layer on the wafer in a photolithography step.

While the thermal expansion coefficient C of the supporting substrate inthe invention may be any value as long as it satisfies the relation“A≦C>B”, more preferably, the value of the thermal expansion coefficientC is adjusted corresponding to the thickness of the nitridesemiconductor layers on the substrate for growing nitride semiconductor.That is, in the case the thickness of the nitride semiconductor layer issignificantly thin as compared with that of the substrate for growingnitride semiconductor (for example, the thickness of the nitridesemiconductor layer is 30 μm or thinner), the thermal expansioncoefficient C of the supporting substrate is preferably adjusted to beapproximately same as the thermal expansion coefficient A of thesubstrate for growing nitride semiconductor. In this case, the thermalexpansion coefficient C of the supporting substrate is not necessarilycompletely same as the thermal expansion coefficient A of the substratefor growing nitride semiconductor, but the thermal expansion coefficientC of the supporting substrate is satisfactory to be within ±10% of thethermal expansion coefficient A of the substrate for growing nitridesemiconductor. On the other hand, in the case the thickness of thenitride semiconductor layer is thick (for example, the thickness of thenitride semiconductor layer exceeds 30 μm), the thermal expansioncoefficient C of the supporting substrate is changed to be closer to thethermal expansion coefficient B of the nitride semiconductor layer fromthe thermal expansion coefficient A of the substrate for growing nitridesemiconductor, depending on the thickness of the nitride semiconductorlayer.

In the fabrication method of the first embodiment of the invention, theforegoing first bonding layer preferably includes an ohmic electrodelayer formed immediately above the p-type nitride semiconductor layers.Further, it is preferable for the first bonding layer to have a firsteutectic-forming layer and the second bonding layer to have a secondeutectic-forming layer. At the time of bonding, the metals respectivelycomposing the first and the second eutecticforming layers are diffusedto form an eutectic and therefore the bonding force can be increased.

Next, the fabrication method of the second embodiment is a fabricationmethod of a nitride semiconductor device comprises:

(a) growing an under layer including a nitride semiconductor having acharacteristic to absorb the light emitted from said device on one mainface of a substrate for growing that has two mutually opposed mainfaces;

(b) forming at least one or more n-type nitride semiconductor layers, anactive layer having a quantum well structure including at least a welllayer of Al_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0≦b≦1, a+b≦1) and a barrierlayer of Al_(c)In_(d)Ga_(1-c-d)N, (0≦c≦1, 0≦d≦1, c+d≦1), and one or morep-type nitride semiconductor layers as active layer on said under layer;(c) bonding a supporting substrate to the surface of said p-type nitridesemiconductor layers; and(d) removing said substrate for growing and said under layer.

With the respect to a nitride semiconductor device, for example, for aUV region with wavelength of 380 nm or shorter, under layers including abuffer layer of Ga_(e)Al_(1-e)N, (0<e<1) and a high-temperature-grownlayer of either undoped GaN or GaN doped with an n-type impurity may beformed. The under layers have an effect to improve the crystallinity ofthe nitride semiconductor to be grown thereon. Further, by employingeither undoped GaN or GaN doped with an n-type impurity as thehigh-temperature-grown layer, the crystallinity of the nitridesemiconductor layer to be grown thereon can be significantly improved.In order to obtain a nitride semiconductor device with goodcrystallinity, it is preferable to grow a GaN layer at a hightemperature on the substrate for growing intervening a buffer layer inbetween. If the active layer or the like is grown without growing suchunder layer, the crystallinity will be considerably inferior and aresulting nitride semiconductor device will be defective in theluminescence output and therefore not practically usable.

Although a nitride semiconductor device with good crystallinity can beobtained by forming such a high-temperature-grown layer such as GaN, aportion of luminescence from the active layer is absorbed in the GaNlayer attributed to the self-absorption of GaN in the UV region. Thisresults in decrease of the luminescence output. In the invention, thesubstrate for growing nitride semiconductor and the under layerincluding GaN are removed after bonding the conductive supportingsubstrate. Therefore, the self-absorption can be suppressed whilekeeping the crystallinity of the nitride semiconductor composing thedevice to be excellent.

When removing the under layer, the under layer is not necessarilyremoved completely if it is removed to such extent that theself-absorption of the luminescence can be suppressed sufficiently. Forexample, in the above-mentioned example, if the thickness of the GaNlayer absorbing the luminescence is as thin as 0.1 μm or thinner(preferably, as thin as 0.01 μm or thinner), the self-absorption will besuppressed sufficiently. For example, in the case the thickness of theGaN layer is smaller than 0.1 μm, about 70% of the light can be pulledout from the device. In the case of the thickness is smaller than 0.01μm, about 96% of the generated light can be pulled out. To remove thesubstrate for growing, it is preferable, for example, to radiateelectromagnetic wave to the entire face of the other main face of thesubstrate for growing. To remove the buffer layer and thehigh-temperature-grown layer, it is preferable, to example, etch orgrind the buffer and high-temperature-grown layer after removing thesubstrate for growing.

The under layer in the second embodiment of the invention is not limitedto the case it includes GaN. The under layer may be a layer whichimproves the crystallinity of the n-type nitride semiconductor layers tobe grown thereon and contains a nitride semiconductor self-absorbing theluminescence of the active layer. For example, even in the case a slightamount of In or Al is added to GaN, if the content of In or Al issignificantly less than that of the layer to be grown thereon, theeffect to improve the crystallinity can be attained. Removal of theunder layer together with the substrate for growing nitridesemiconductor after the device structure is formed through the underlayer in such a manner suppresses the self-absorption while thecrystallinity of the nitride semiconductor composing the device beingkept excellent.

The phrase “a nitride semiconductor that absorbs the light emitted formthe device” means a nitride semiconductor having a band gap energy closeto or smaller than that of the active layer and accordingly absorbingthe luminescence to a non-negligible extent. For example, in the casethe band gap energy of a nitride semiconductor is smaller than acriteria value that is 0.1 eV larger than the luminescence peak, asshown in the following equation, it absorbs the luminescence of theactive layer.

(The band gap energy of the nitride semiconductor layer)≦(theluminescence peak energy +0.1 eV)

The relation of the band gap energy of the nitride semiconductor withthe composition can be assumed as the case that the bowing parameter isset to be 1. For example, the band gap energy of a nitride semiconductorof ternary mixed crystal A_(1-x)B_(x)C can be expressed as the followingequation: E_(G)(A_(1-x)B_(x)C)=(1−x)E_(G,AC+x)E_(G,BC)−(1−x)x whereinE_(G,AC) and E_(G,BC) denote the band gap energy for the binary mixedcrystal AC and BC, respectively.

Also, the phrase “an under layer for improving the crystallinity of then-type nitride semiconductor” means an under layer by which thecrystallinity of the n-type nitride semiconductor in a device isincreased as compared with that of a nitride semiconductor device havingthe same layer structure except that the under layer is not formed. Ingeneral, any under layer with a composition easy to have goodcrystallinity as compared with that of a layer to be grown thereon, theunder layer can be said to have a capability of improving thecrystallinity. In the case of the nitride semiconductor, a ternary mixedcrystal is easier to have good crystallinity than a quaternary mixedcrystal, and a binary mixed crystal is easier than a ternary mixedcrystal. In the case of between identical quaternary mixed crystals orbetween identical ternary mixed crystals, those with a smaller In or Alcomposition ratio are easier to have good crystallinity.

The fabrication methods (hereinafter, referred to as the fabricationmethod of the invention) of the first embodiment and the secondembodiment of the invention may be combined with each other. While thefabrication method of the invention is applicable for devices comprisingan active layer of Al_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0≦b≦1, a+b≦1) andemitting a luminescence of a variety of wavelength, the presentinvention is especially effective in a UV region with 380 nm or shorterwavelength. A nitride semiconductor device preferable for a shortwavelength region, i.e. 380 nm or shorter wavelength can be fabricatedby forming an active layer having a quantum well structure containing atleast a well layer of a quaternary mixed crystal of InAlGaN and abarrier layer containing at least Al-containing nitride semiconductor.Since the well layer is made of the quaternary mixed crystal of InAlGaNin the above-mentioned active layer, the degrade of the crystallinitycan be suppressed while the number of the component elements beingsuppressed to the minimum. Therefore the light emitting efficiency canbe improved. Further, use of the nitride semiconductor containing atleast Al for the barrier layer widens band gap energy than that of thewell layer, makes formation of the active layer having a quantum wellstructure with the band gap energy corresponding to the luminescencewavelength possible and keeps the crystallinity of the active layerexcellent.

In the fabrication method of the invention, a composition-graded layermay be formed further on the high-temperature-grown layer. Thecomposition-graded layer is for moderating the lattice inconformity withthe nitride semiconductor to be grown thereon and the composition ratiois gradually changed from the composition of the high-temperature-grownlayer to the composition of the nitride semiconductor layer to be grownthereon. For example, in the case that the high temperature grown layeris made of an undoped GaN and the nitride semiconductor layer to begrown thereon is an n-type clad layer of Al_(v)Ga_(1-v)N, thecomposition-graded layer is formed in which the mixed crystal ratio ofAl is gradually increased from GaN to Al_(v)Ga_(1-v)N. Thecomposition-graded layer is especially effective for LED emittingluminescence in UV region in that defects in the nitride semiconductorlayers are extremely reduced and their crystallinity is remarkablyimproved. Also, the composition-graded layer is effective for LED ofwhich n-type clad layer has a relatively high Al content (for example,5% or more). By the composition-graded layer, the lattice mismatch atthe interface of the n-type clad layer is reduced and the crystallinityof the semiconductor layers is remarkably improved. Further, thecomposition-graded layer may be modulation-doped layer in which animpurity for determining the conductivity is added in graded state. Forexample, in the case the nitride semiconductor layer to be grown thereonis a Si-doped Al_(v)Ga_(1-v)N, the composition-graded layer may have astructure in which the impurity concentration is changed from un-dopedstate to the Si concentration of an n-type clad layer. This reducesdefects and improves the crystallinity of the nitride semiconductorlayer.

Further, in fabrication method of the invention, a coating layercontaining a phosphor substance is preferably formed in at least aportion of the surface of a nitride semiconductor device.

A nitride semiconductor device of the third embodiment of the invention,especially a nitride semiconductor device for the UV region with 380 nmor shorter wavelength, comprises:

a substrate having two opposed main faces and having a thermal expansioncoefficient higher than that of a nitride semiconductor;

a bonding layer placed on one main face of the substrate and includingan eutectic layer;

one or more p-type nitride semiconductor layers placed on the bondinglayer;

an active layer including at least a well layer ofAl_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0<b≦1, a+b<1) and a barrier layer ofAl_(c)In_(d)Ga_(1-c-d)N, (0<c≦1, 0≦d≦1, c+d<1) and placed on said p-typenitride semiconductor layers; and

one or more n-type nitride semiconductor layers containing Al and placedon said the active layer. In this case, the bonding layer is composed ofsaid first bonding layer and second bonding layer. In the bonding layer,the first eutectic forming layer and the second eutectic forming layerare mutually diffused to form an eutectic layer.

The above-mentioned substrate is preferably conductive and morepreferably contains both a metal with high electric conductance and ametal with high hardness. Preferably, a metal material with a highconductivity and a high thermal expansion coefficient is combined with ametal material with a high hardness and a low thermal expansioncoefficient are compounded, so that a substrate with a high conductivityand a thermal expansion coefficient higher than that of the nitridesemiconductor layers can be composed. As the metal material with a highhardness and a low thermal expansion coefficient, for example, Ag, Cu,Au, Pt and the like can be exemplified. As the metal material with ahigh hardness and a low thermal expansion coefficient, for example, W,Mo, Cr, Ni and the like can be exemplified.

In the case the metal material with a high conductivity and a highthermal expansion coefficient and the metal material with a highhardness and a low thermal expansion coefficient do not or hardly form asolid-solution, the substrate can be made of composites of these metalmaterials. Employing a composite metal material makes it possible to usethe combination of the metal materials having significantly differentproperties from each other. Therefore, a supporting substrate havingboth a desired thermal expansion and high conductivity can be obtained.Further, the substrate may be made of a composite of a metal materialhaving high conductivity and a high thermal expansion coefficient with aceramic material having high hardness an a low thermal expansioncoefficient such as diamond. In such a manner, a supporting substratehaving a desired thermal expansion coefficient while maintaining thehigh conductivity may be obtained.

A nitride semiconductor device of the fourth embodiment of the inventioncomprises:

a substrate having two opposed main faces;

a bonding layer placed on one main face of the substrate and includingan eutectic layer;

one or more p-type nitride semiconductor layers placed on said bondinglayer;

an active layer including at least a well layer ofAl_(a)In_(b)Ga_(1-a-b)N, (0<a≦1, 0<b≦1, a+b<1) and a barrier layer ofAl_(c)In_(d)Ga_(1-c-d)N, (0<c≦1, 0≦d≦1, c+d≦1) and placed on said p-typenitride semiconductor layers; and

n-type nitride semiconductor layers placed on said active layer and madeof a nitride semiconductor which does not substantially absorb the lightemitted from said active layer.

The phrase “a nitride semiconductor which does not substantially absorbthe light emitted from the active layer” means a nitride semiconductorhaving a band gap energy is so large that the self-absorption ofluminescence from the active layer can be suppressed sufficiently. Forexample, in the case the band gap energy of a nitride semiconductor islarger than a criteria value that is 0.1 eV larger than the luminescencepeak energy, self-absorption of the luminescence by the nitridesemiconductor can be negligible. Also, in the case the nitridesemiconductor contains a layer with a band gap energy causingself-absorption, if the thickness of the layer is 0.1 μm or thinner(more preferably 0.01 μm or thinner), the self-absorption of theluminescence can be negligible.

With respect to the nitride semiconductor devices of the third and thefourth embodiments (hereinafter referred to as a nitride semiconductordevice of the invention), especially in a nitride semiconductor devicein a UV range with wavelength of 380 nm or shorter, the above-mentionedp-type nitride semiconductor layers may includes a p-type contact layerof Al_(f)Ga_(1-f)N, (0<f<1). The p-type contact layer is preferable hasa graded composition in which a p-type impurity concentration is highand a mixed crystal ratio of Al is low in the conductive substrate side.In this case, the graded composition may be a continuously changedcomposition or a composition intermittently changed in step by step.

Further, with respect to a nitride semiconductor device of theinvention, especially a nitride semiconductor device in a UV range withwavelength of 380 nm or shorter, the above-mentioned p-type contactlayer may be composed of two layers; a first p-type contact layer ofAl_(g)Ga_(1-g)N, (0<g<0.05) formed in the conductive electrode side anda second contact layer of Al_(h)Ga_(1-h)N, (0<h<0.1) formed in theactive layer side; and the first contact layer may have a higher p-typeimpurity concentration than that of the second contact layer. However,the p-type contact layer is not necessarily to be Al_(h)Ga_(1-f)N,(0<f<1) but may be GaN. That is because the p-type contact layer is forforming a p-electrode and may be any material if it has an ohmic contactwith the p-electrode. Also, since the contact layer may generally bethin as compared with a high-temperature-grown layer, even if GaN isused for the p-type contact layer, decrease of the luminescentoutputting efficiency owing to the self-absorption of the layer is notso significant.

With respect to a device of the invention, especially a light emittingdevice, luminescence with a variety of wavelength values can be emittedby forming a coating layer containing a phosphor substance which absorbsa portion or entire luminescence from the active layer ofAl_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0≦b≦1, a+b≦1) and emits luminescence ofdifferent wavelength values in the nitride semiconductor device on aconductive substrate. Especially, by adding YAG as a phosphor material,white light emission is made possible and the range of the deviceapplication is considerably wider to such as a light source forluminaries and the like.

With respect to the phosphor substance, the materials which absorbvisible light rays and emits light rays with different wavelength valuesare limited. However, there are many kinds of materials which absorb UVrays and emits light rays with different wavelength values areavailable. Accordingly, with the light emitting device emitting UVlight, an adequate material can be selected from a variety of materials.Also, phosphor substances which absorb UV rays have a high lightconversion efficiency as compared with a visible light conversionefficiency. Especially, with respect to white light, color-renderingwhite light can be obtained and the application possibility is furtherwidened. The invention provides, as a nitride semiconductor deviceemitting luminescence in UV range, a nitride semiconductor lightemitting device with little self-absorption and also provides a lightemitting device for emitting white light with an extremely highconversion efficiency by forming a coating with a phosphor substance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating fabricationprocess in one embodiment of a fabrication method of a nitridesemiconductor device according to the invention.

FIGS. 2A and 2B show schematic cross-sectional views illustrating thewarp of a wafer in the case the thermal expansion coefficients A, B, andC of a substrate for growing nitride semiconductor, nitridesemiconductor layers, and a supporting substrate, respectively, have arelation of C>A>B.

FIGS. 3A to 3D show schematic cross-sectional views illustrating thewarp of a wafer in the case the thermal expansion coefficients A, B, andC of a substrate for growing nitride semiconductor, nitridesemiconductor layers, and a supporting substrate, respectively, have arelation of A>B>C.

FIGS. 4A to 4D show schematic cross-sectional views illustrating thewarp of a wafer in the case the thermal expansion coefficients A, B, andC of a substrate for growing nitride semiconductor, nitridesemiconductor layers, and a supporting substrate, respectively, have arelation of A≦C>B.

FIGS. 5A and 5B show a plane view and a cross-sectional viewillustrating one embodiment of a nitride semiconductor device accordingto the invention.

FIGS. 6A to 6C show a perspective view, a top view and a cross-sectionalview illustrating one package to be employed for a nitride semiconductordevice according to the invention.

FIGS. 7A to 7C show a perspective view, a top view and a cross-sectionalview illustrating another package to be employed for a nitridesemiconductor device according to the invention.

FIG. 8 is a process drawing illustrating another embodiment differentfrom that in FIG. 1.

FIG. 9 shows a schematic cross-sectional view illustrating a layerstructure of the nitride semiconductor device of the embodiment shown inFIG. 8.

FIG. 10 shows a process drawing illustrating another embodimentdifferent from that in FIG. 8.

FIGS. 11A and 11B show the characteristics of the nitride semiconductordevice according to Example 10.

FIG. 12 is a graph comparing the characteristics of the nitridesemiconductor device of Example 10 and a conventional nitridesemiconductor device.

FIG. 13 is a schematic cross-sectional view showing a nitridesemiconductor device of another embodiment different from that shown inFIG. 5.

FIG. 14 is a schematic cross-sectional view showing an embodiment inwhich a coating layer containing a phosphor is formed.

FIG. 15 is a schematic cross-sectional view showing an embodiment inwhich dimples are formed in the surface of an n-type nitridesemiconductor layer.

FIGS. 16A to 16L show process drawings illustrating a fabrication methodof a nitride semiconductor device according to Example 23.

FIGS. 17A and 17B are schematic cross-sectional views showing a nitridesemiconductor laser according to Example 23.

FIG. 18 is a schematic cross-sectional view showing the structure of aconventional nitride semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The application is based on applications Nos. 2002-198761, 2002-218199,2002-276309, 2003-4919 filed in Japan, the content of which isincorporated herein by reference.

While the invention has been described with reference to specificembodiments, various modifications and applications may occur to thoseskilled in the art. Such modifications and substitutions can be madewithout departing from the spirit and scope of the present invention asdefined by the appended claims.

FIG. 1 is a schematic cross-sectional view illustrating fabricationprocess of a fabrication method of a nitride semiconductor deviceaccording to the invention. An under layer 2 composed of a buffer layer3 and a high temperature grown layer 4 is formed on the surface of asubstrate for growing nitride semiconductor 1 (FIG. 1A). Next, an n-typeclad layer 5, an active layer 6, a p-type clad layer 7, a p-type contactlayer 8, and a first bonding layer 9 composed of one or more metallayers are formed on the under layer 2 (FIG. 1B). Here, in the firstbonding layer 9, after a p-electrode is formed on the p-type contactlayer 8, annealing treatment is carried out in order to obtain ohmiccontact. Next, a conductive substrate 10 on whose surface a secondbonding layer 11 composed of one or more metal layers is formed islaminated on the substrate for growing nitride semiconductor 1 while thefirst bonding layer 9 and the second bonding layer 11 being set face toface and bound by pressing and heating.

Next, the substrate for growing nitride semiconductor 1 bound to theconductive substrate 10 is set in a polishing apparatus and thesubstrate for growing nitride semiconductor 1 is subjected to lapping toremove the substrate for growing nitride semiconductor 1 and the underlayer 2 to expose the n-type clad layer 5 (FIG. 1D).

Next, after the surface of the exposed n-type clad layer 5 is polished,an n-electrode 12 is formed on the n-type clad layer 5, and on the otherhand, a pad electrode 13 for a p-electrode is formed on the entire faceof the conductive substrate 10. Next, light emitting device is separatedinto chips by dicing to obtain a light emitting device comprisingnitride semiconductor layers laminated on the conductive substrate andthe electrode formed on the conductive substrate (FIG. 1E).

In the method of the invention, as the substrate for growing nitridesemiconductor, sapphire having any of C plane, R plane, and A plane as amain face, spinel (an insulating substrate of such as MgAl₂O₄), SiC, Si,and an oxide substrate having lattice conformity with that of thenitride semiconductor can be exemplified. Sapphire and spinel arepreferable.

In the case nitride semiconductor layers are laminated on a substratefor growing nitride semiconductor, ELOG (Epitaxially Lateral Overgrowth)is carried out on the under layer to obtain the nitride semiconductorwith improved crystallinity. Practically, the under layer is grown onthe substrate for growing nitride semiconductor and a plurality of masksin stripes are formed on the under layer and nitride semiconductor isselectively grown from the apertures of the masks and a nitridesemiconductor layer (a laterally grown layer) formed by growthaccompanied with the grown in the lateral growth is formed. Since thethrough dislocation is suppressed in the laterally grown layer, thecrystallinity of the nitride semiconductor to be formed on the laterallygrown layer can be improved.

As the substrate for growing nitride semiconductor, a substrate producedfrom a material to be a substrate for growing which is off-angles,preferably off-angled in steps, in the main face is preferable. If anoff-angled substrate is used, then three-dimensional growth in thesurface does not take place and step growth occurs to make the surfaceflat easily. Further, if the direction along the steps (the stepdirection) of a sapphire substrate which is off-angles in steps isadjusted to be perpendicular to the A plane of the sapphire, the stepface of the nitride semiconductor coincides with the resonance directionof laser and diffused reflection of the laser beam owing the surfaceroughness can be suppressed preferably.

Further, as the supporting substrate to be bound to the p-type nitridesemiconductor layer, for example, a semiconductor substrate of asemiconductor of such as Si, SiC and the like, or a single metalsubstrate or a metal substrate of a composite of two or more metalswhich do not or scarcely form solid-solution can be employed. A metalsubstrate is preferably used because a metal substrate is excellent inmechanical properties as compared with a semiconductor substrate andeasy to be elastically or plastically deformed and hard to be cracked.Further, as the metal substrate, those consisting of one or more metalsselected from highly conductive metals such as Ag, Cu, Au, Pt and thelike and one or more metals selected from high hardness metals such asW, Mo, Cr, Ni and the like can be employed. Further, as the metalsubstrate, a composite of Cu—W or Cu—Mo is preferably used because itcontains Cu with a high thermal conductivity and therefore is excellentin heat releasing property. Further, in the case of the Cu—W composite,the content x of Cu is preferably 0<x≦30% by weight and in the case ofthe Cu—Mo composite, the content x of Cu is preferably 0<x≦50% byweight. Further, composites of a metal and a ceramic such as Cu-diamondmay be used. The thickness of the substrate to be bound to the p-typenitride semiconductor layer is preferably 50 to 500 μm to increase theheat releasing property. The heat releasing property can be madeexcellent by making the supporting substrate thin in the above-mentionedrange. The supporting substrate may have an uneven structure in the faceto be stuck to the nitride semiconductor or on the opposed face.

It is preferable to select a material for the supporting substrate so asto satisfy A≦C>B wherein the A, B, and C denote thermal expansioncoefficients of the substrate for growing nitride semiconductor, thenitride semiconductor layers, and the supporting substrate. In the casethe supporting substrate is a metal composite, the thermal expansioncoefficient can be adjusted to be as desired by controlling thecomposition ratio of the metal material made to be the composite. Forexample, in the case of composing the supporting substrate using acomposite of Cu and Mo, the thermal expansion coefficient of Cu is about16×10⁻⁶ K⁻¹ and the thermal expansion coefficient of Mo is about 5×10⁻⁶K⁻¹. Accordingly, in the case the thermal expansion coefficient of thesupporting substrate is needed to be low, the composition ratio of Cu inthe composite is adjusted to be low and in the case the thermalexpansion coefficient is needed to be high, the composition ratio of Cuin the composite is adjusted to be high.

Further, it is preferable for the first layer to have ohmic contact withthe p-type nitride semiconductor layer and comprise the p-electrodehaving a high reflectivity and contact with the p-type nitridesemiconductor layer. For the p-electrode, a metal material containing atleast one metal selected from a group consisting of Ag, Rh, Ni, Au, Pd,Ir, Ti, Pt, W, and Al is preferably used. Rh, Ag, Ni—Au, Ni—Au—RhO orRh—Ir is preferable to be used and Rh is furthermore preferable to beused. In this case, since the p-electrode is formed on the p-typenitride semiconductor layer with a high resistance as compared with then-type nitride semiconductor layer, the p-electrode is preferably formedon approximately entire surface of the p-type nitride semiconductorlayer. The thickness of the p-electrode is preferably 0.05 to 0.5 μm.

Further, an insulating protection film is preferable to be formed on theexposed face of the p-type nitride semiconductor layer bearing thep-electrode of the first bonding layered. A monolayer film or amulti-layer film of SiO₂, Al₂O₃, ZrO₂, TiO₂ and the like may be used fora material of the protection film. Further, a metal film of such as Al,Ag, Rh and the like with a high reflectivity may be formed on theprotection film. The reflectivity is increased by the metal film and thelight outputting efficiency can be improved.

Further, it is also preferable to form a first eutectic forming layer onthe p-electrode of the first bonding layer and a second eutectic forminglayer on the main face of the conductive substrate in the second bondinglayer. The first and second eutectic forming layers are layers formingeutectic by mutual diffusion at the time of bonding and Au, Sn, Pd, In,Ti, Ni, W, Mo, Au—Sn, Sn—Pd, In—Pd, Ti—Pt—Au, Ti—Pt—Sn and the like arepreferably used. The first and second eutectic forming layers are morepreferably made of metals such as Au, Sn, Pb, In and the like. Thecombination of the first and second eutectic forming layers ispreferably Au—Sn, Sn—Pd or In—Pd. Further preferable combination is Snfor the first eutectic forming layer and Au for the second eutecticforming layer.

Further, a closely sticking layer and a barrier layer are preferablyformed from the p-electrode side between the first eutectic forminglayer of the first bonding layer and the p-electrode. The closelysticking layer is a layer for assuring the highly close adhesion to thep-electrode and is preferably made of a metal of Ti, Ni, W or Mo. Thebarrier is a layer for preventing a metal composing the first eutecticforming layer from diffusing to the closely sticking layer and ispreferably made of Pt or W. In order to further prevent diffusion of themetal composing the first eutectic forming layer to the closely stickinglayer, a Au film with a thickness of about 0.3 μm may be formed betweenthe barrier layer and the first eutectic forming layer. Incidentally,the above-mentioned closely sticking layer, barrier layer, and Au filmare preferably formed between the second eutectic forming layer and theconductive substrate.

As a combination of the closely sticking layer, barrier layer, andeutectic forming layer, for example, Ti—Pt—Au, Ti—Pt—Sn, Ti—Pt—Pd orTi—Pt—AuSn, W—Pt—Sn, RhO—Pt—Sn, RhO—Pt—Au, RhO—Pt—(Au, Sn) and the likecan be exemplified. These metal films are alloyed by the eutectic andform a conductive layer in a step thereafter. The first and secondeutectic forming layers are preferably different from each other. Thereason for that is because an eutectic can be formed at a lowtemperature and the melting point after eutectic formation can beincreased.

The temperature at the time of bonding the laminate for bonding and theconductive substrate by pressing and heating is preferably 150 to 350°C. Because if it is 150° C. or higher, diffusion of the metals of theeutectic forming layers is promoted to form an eutectic with evendensity distribution and improve the adhesion strength of the laminatefor bonding and the conductive substrate. If it is higher than 350° C.,the metals of the eutectic forming layers are diffused to the barrierlayer and further to the closely sticking layer to result inimpossibility of obtaining high adhesion strength.

At the time of lamination, the first bonding layer will have thefollowing structure. That is, p-electrode/Ti—Pt—AuSn—Pt—Ti/conductivesubstrate, p-electrode/RhO—Pt—AuSn—Pt—Ti/conductive substrate,p-electrode/Ti—Pt—PdSn—Pt—Ti/conductive substrate,p-electrode/Ti—Pt—AuSn—Pt—RhO/conductive substrate, and the like.Accordingly, a hardly peeling alloy can be formed. Lamination at a lowtemperature is thus made possible by forming the conductive layer of theeutectic and the adhesion force is also made high. Lamination at a lowtemperature is effective to moderate the warping.

To remove the substrate for growing nitride semiconductor after bondingthe conductive substrate, polishing, etching, electromagnetic waveradiation or a method combining these methods can be employed. In thecase of electromagnetic wave radiation, for example laser is employedfor the electromagnetic wave and laser beam is radiated to the entiresurface of the face where no under layer of the substrate for growingnitride semiconductor is formed to remove the substrate for growingnitride semiconductor and the under layer by decomposition of the underlayer after bonding of the conductive substrate. Also, a desired filmcan be exposed by removing the substrate for growing nitridesemiconductor and the under layer and then subjecting the surface of theexposed nitride semiconductor layer to CMP treatment. Accordingly,removal of a damaged layer, the thickness and the surface roughness ofthe nitride semiconductor layers can be adjusted.

To radiate electromagnetic wave, the following method can be employedtoo. That is, an under layer of a nitride semiconductor is formed on thesubstrate for growing nitride semiconductor and then partially etched tothe substrate for growing nitride semiconductor to form projected andrecessed parts, and after that, ELOG is carried out on the under layerhaving the projected and recessed parts to form a laterally grown layer.Next, after an n-type nitride semiconductor layer, an active layer, anda p-type nitride semiconductor layer are successively formed on thelaterally grown layer, a conductive substrate is bound to the p-typenitride semiconductor layer. Finally, laser is radiated to the entiresurface of the face where no under layer of the substrate for growingnitride semiconductor is formed to remove the substrate for growingnitride semiconductor and the under layer by decomposition of the underlayer. By the method, N₂ gas generated by the decomposition of thenitride semiconductor at the time of electromagnetic radiation is spreadto the voids among the foregoing projected and recessed parts and thelaterally grown layer to prevent cracking of the substrate for growingnitride semiconductor owing to gas pressure and further prevent scoopingdamages of the under layer attributed to the cracking and consequentlyobtain a nitride semiconductor substrate with excellent plane state andcrystallinity. As compared with a method by polishing, the work processcan be simplified to result in improvement of the production yield.

Further, to improve the luminescence outputting efficiency, as shown inFIG. 15, projected and recessed parts (dimples) may be formed by RIE ofthe exposed face of the n-type nitride semiconductor layer 5 after theremoval of the substrate for growing nitride semiconductor. Theprojected and recessed part (dimples) formed side is a luminescenceoutputting side of the nitride semiconductor. Owing to the projected andrecessed part formation in the surface, the light rays which cannot comeout due to the total reflection can be emitted by angular change of thelight rays by the projected and recessed face. That is, the emittedlight rays can be emitted by diffused reflection in the projected andrecessed parts and light rays which are conventionally totally reflectedare led upward and outputted to the outside of the device. It can beexpected to improve the output by the formation of the projected andrecessed parts 1.5 times as high as that without projected and recessedpart formation. The plane shape of the projected and recessed parts ispreferably round or polygonal such as hexagonal or triangular. The planeshape of the projected and recessed parts is also preferable to be instripes, lattice, rectangular. In order to improve the luminescenceoutputting efficiency, pattern pitches of the projected and recessedparts are preferably as fine as possible. Further, the cross-sectionalshape of the projected and recessed parts is preferably gentlycorrugated shape rather than composed of flat and straight lines. Theluminescence outputting efficiency can be improved by making thecross-sectional shape of the projected and recessed parts be acorrugated shape as compared with that in the case the cross-sectionalshape is made angular. Further, the depth of the recessed parts ispreferably 0.2 to 3 μm, more preferably 0.1 to 1.5 μm. Because if thedepth of the recessed parts is too shallow, the effect to improve theluminescence outputting efficiency cannot be sufficient and if it is toodeep, the resistance in the lateral direction is increased. In the casethe recessed parts are formed by scooping in round or polygonal shape,the output can be improved while the resistance value being kept low.

Further, multilayer electrodes of such as Ti—Al—Ni—Au or W—Al—W—Pt—Aucan be employed for the n-electrode to be formed on the exposed face ofthe n-type nitride semiconductor layer. The thickness of the n-electrodeis preferably 0.1 to 1.5 μm. An insulating protection film of such asSiO₂, Al₂O₃, ZrO₂, TiO₂ and the like is preferably formed so as to coatthe exposed face other than the n-electrode.

The p-electrode and n-electrode are not limited in the shapes and thesizes as long as the p-electrode is formed in one main face of thenitride semiconductor device and the n-electrode is formed in the othermain face. Preferably, both electrodes are arranged on the opposite toeach other so as not to be overlapped when observed in the layereddirection of the nitride semiconductor layers. By this arrangement, inthe case of a face-down structure, emitted light can be efficientlyoutputted without being shielded by the n-electrode. For example, in thecase that the p-electrode is formed over almost the entire surface ofthe p-type nitride semiconductor layer, the n-electrode may be dividedinto two or four and placed in the corner portions of the n-type nitridesemiconductor layer. Alternatively, the n-electrode may be formed in alattice-like shape over the entire face of the n-type semiconductorlayer, or may be formed at the corner portions of the n-typesemicondutor in a lattice-like shape.

With reference to FIG. 5, the preferable shapes of the p-electrode andthe n-electrode will be described in details.

FIGS. 5A and 5B show a plane view and a cross-sectional viewillustrating one nitride semiconductor device according to theinvention. A conductive layer 15 of an eutectic from a first and asecond bonding layers, a p-electrode 16, and a nitride semiconductor 17are successively formed on a supporting substrate 10. An n-electrode 12is formed on the nitride semiconductor 17. The n-electrode 12 comprisespad electrode formation regions 12 a in the corner parts in a diagonalline of a chip and is spread like a net between the pad electrodeformation regions 12 a. The electrode 12 is formed in net-like orlattice-like shape on the approximately entire face in the lightemitting range, so that current can flow evenly in the nitridesemiconductor layer 17. The pad electrode formation region 12 a may notbe limited to two regions on the diagonal line but formed in all fourcorners. The p-electrode 16 and the n-electrode 12 are formed in amanner that both electrodes are not overlapped when being observed fromthe top face of the chip. A protection film 19 is formed on then-electrode 12. The protection film 19 may be formed not only on thenitride semiconductor layer 17 but also on the p-electrode 16 other thanon the pad electrode formation regions 12 a of the n-electrode.

As illustrated in FIG. 5B, apertures are formed in the p-electrode 16adjacent to the nitride semiconductor 17 and an insulating protectionfilm 20 is formed in the inside of the apertures. A monolayer film or amultilayer film of SiO₂, Al₂O₃, ZrO₂, TiO₂ and the like may be used fora material of the protection film 20. Formation of the insulatingprotection film 20 prevents short-circuiting and improves the yield andthe reliability. The protection film 20 preferably has a double layerstructure with a reflection film (not illustrated). For example,formation of a reflection film (not illustrated) of Al, Ag, Rh or thelike in a thickness of not thinner than 500 Å and not thicker than 2,000Å in the side where the protection film 20 does not contact with thenitride semiconductor 17 improves the output efficiently of lighttransmitted in the lateral direction. The reflection film may be formedin the supporting substrate 10 side or in the nitride semiconductor 17side.

Incidentally, the electric connection of the p-electrode 16 and theoutside of the device can be formed through the conductive layer 15. Forexample, in the case the supporting substrate 10 is conductive, a wireis connected to the rear face of the supporting substrate 10, so thatthe electric connection of the p-electrode 16 and its outside part canbe formed through the conductive layer 15 and the supporting substrate10. In the case the supporting substrate 10 is not conductive, a wire isconnected to the conductive layer 15 from the chip side face, so thatthe electric connection of the p-electrode 16 and its outside part canbe formed. Further, the supporting substrate 10 and the conductive layer15 may be made to be wider than the nitride semiconductor layer 17 and awire may be connected to the part of the conductive layer 15 which isnot coated with the nitride semiconductor layer 17.

Hereinafter, a practical constitution of a nitride semiconductor deviceaccording to the invention will be described.

(Under Layer)

An under layer 2 can be composed of at least one or more nitridesemiconductor layers and it is preferably composed of a buffer layer 3grown at a low temperature on a substrate for growing nitridesemiconductor 1 and a high temperature grown layer 4 grown at a hightemperature on the buffer layer.

The buffer layer 3 is made of a nitride semiconductor ofGa_(i)Al_(1-i)N, (0<i≦1) and use of a nitride semiconductor with a lowAl ratio, preferably GaN, improves the crystallinity of a nitridesemiconductor to be grown on the buffer layer. The thickness of thebuffer layer is preferably 0.002 to 0.5 μm, more preferably 0.005 to 0.2μm, and further more preferably 0.01 to 0.02 μm.

The growing temperature of the buffer layer is preferably 200 to 900°C., more preferably 400 to 800° C.

An un-doped GaN or GaN doped with an n-type impurity is preferable to beused for the high temperature grown layer 4. Generally, since GaN hasexcellent crystallinity, growth of GaN as an under layer improves thecrystallinity of the device structure to be grown thereon. Incidentally,the high temperature grown layer 4 may contain In and Al to an extentthat the crystallinity improvement effect is not lost. The In ratio inthe high temperature grown layer 4 is preferably not higher than 0.01.Also, the Al ratio in the high temperature grown layer 4 is desirablynot higher than 0.01. Especially in the case the high temperature grownlayer 4 contains In, the crystal of the high temperature grown layer 4is softened and therefore, an effect to moderate the strain generated inthe interface to the substrate for growing nitride semiconductor can beobtained. The thickness of the high temperature grown layer ispreferably not thinner than 500 Å, further preferably not thinner than 5μm, and further more preferably not thinner than 10 μm. The growthtemperature of the high temperature grown layer is preferably 900 to1,100° C., more preferably 1,050° C. or higher.

In the case the nitride semiconductor employed for the high temperaturegrown layer 4 self-absorbs the luminescence from the active layer, thehigh temperature grown layer 4 is preferable to be removed finally. Forexample, in the case the band gap energy of the high temperature grownlayer 4 is smaller than (luminescence peak energy +0.1 ev), luminescencefrom the active layer is absorbed by such a high temperature grown layer4, so that removal of the high temperature grown layer 4 results inincrease of the luminous intensity. If the thickness is thin enough tosuppress the self-absorption of the luminescence to a negligible level,for example, a thickness of 0.1 μm or thinner (more preferably, 0.01 μmor thinner), a portion of the high temperature grown layer 4 may beleft.

(n-Type Clad Layer Working also as n-type Contact Layer 5)

Any n-type clad layer 5 is not particularly limited if it has acomposition with a higher band gap energy than that of the active layer6 and is capable of enclosing the carrier in the active layer 6, howeverAl_(j)Ga_(1-j)N, (0<j<0.3) is preferable. Here, 0.1<j<0.2 is furtherpreferable. Although the thickness of the n-type clad layer is notparticularly limited, it is preferably 0.01 to 0.1 μm, furtherpreferably 0.03 to 0.06 μm. The n-type impurity concentration of then-type clad layer is also not limited, however it is preferably 1×10¹⁷to 1×10²⁰/cm³, more preferably 1×10¹⁸ to 1×10¹⁹/cm³. Further, in theinvention, in the case the substrate for growing nitride semiconductor,the under layer and GaN of the high temperature grown layer are removedafter the conductive substrate is bound, a certain thickness is requiredand in such a case the thickness if controlled to be 1 to 10 μm, furtherpreferably 1 to 5 μm. Incidentally, in this embodiment of the invention,the n-type clad layer 5 works also as the n-type contact layer.

(Another Embodiment of n-type Contact Layer)

Any material can be used for the n-electrode in the embodiment of theinvention if the material is capable of having ohmic contact with then-type nitride semiconductor layer. However, in the case the followingsteps (1) to (5) are carried out in the this order in the fabricationprocess as the process of the invention; (1) p-electrode formation; (2)ohmic annealing; (3) bonding the conductive substrate; (4) removing thesubstrate for growing nitride semiconductor; and (5) forming then-electrode; the n-electrode can be formed without the ohmic annealing.Further, if it is tried to carry out ohmic annealing, since an eutecticlayer exists in the bonding layer of the conductive substrate, annealingat a high temperature is difficult to be carried out. Therefore, in thestep of cleaning treatment to be carried out after the n-electrodeformation, the substrate is heated at 150 to 350° C. to result in aproblem that the n-electrode is made thermally unstable. Further, withrespect to a high output light emitting device fabricated in suchfabrication steps, there occurs a problem that the n-electrode is madethermally unstable by the heat at the time of light emission. Further,owing to the thermal instability, the ohmic contact tends to be Schottkycontact.

In the constitution of the invention, since the n-type nitridesemiconductor layer is exposed by removing the substrate for growingnitride semiconductor and the n-electrode is formed in the exposed face,micro cracking is sometimes observed in the exposed face owing topolishing. Occurrence of the cracks not only interferes even flow ofelectric current but also makes it impossible to obtain high bondingstrength between the n-electrode and the n-type nitride semiconductorlayer with a high productivity.

Since the under layer which self-absorbs the luminescence of the activelayer is removed together with the substrate for growing nitridesemiconductor in the embodiment of the invention, the band gap energy ofthe contact layer (the n-type clad layer in this embodiment of theinvention) is high as compared with that of a conventional nitridesemiconductor light emitting device. However, generally, the higher theband gap energy of a semiconductor material becomes, the more difficultit tends to become to have ohmic contact.

As a preferable embodiment to solve such problems as described above,the nitride semiconductor device of the invention comprises then-electrode contacting with the n-type nitride semiconductor layer andthe n-type nitride semiconductor layer is composed of at least twolayers; a first n-type nitride semiconductor layer doped with an n-typeimpurity as a layer contacting with the n-electrode and a second n-typenitride semiconductor layer un-doped or doped with an n-type impurity ina less amount than that of the first n-type nitride semiconductor layerin the active layer side rather than the first n-type nitridesemiconductor layer. For example, as shown in FIG. 10, the n-type cladlayer 5 working also as an n-type contact layer is divided into a firstn-type nitride semiconductor layer 5 a and a second n-type nitridesemiconductor layer 5 b.

The doping amount of the n-type impurity is not lower than 3×10¹⁸/cm³and not higher than 1×10²⁰/cm³ for the first n-type nitridesemiconductor layer 5 a, and not lower than 1×10¹⁷/cm³ and lower than3×10¹⁸/cm³ or none for the second n-type nitride semiconductor layer 5b, respectively. The respective n-type nitride semiconductor layers mayhave similar or dissimilar compositions, however they are preferablyformed with compositions in the above-mentioned range of the preferablecomposition of the n-type clad layer 5.

The reason for the less n-type impurity concentration of the secondn-type nitride semiconductor layer 5 b nearer to the active layer is toavoid an inverse effect of the n-type impurity on the active layer.Since the invention involves particular steps such as a step ofpress-bonding by heating at the time of bonding a conductive supportingsubstrate and a step of removing a portion of the n-type nitridesemiconductor layer by polishing and laser radiation, if the a largequantity of the n-type impurity is contained in the n-type nitridesemiconductor layer near the active layer, the impurity causes inverseeffects on the active layer. Therefore, the first n-type nitridesemiconductor layer 5 a forming the n-electrode is doped in a highconcentration, whereas the second n-type nitride semiconductor layer 5 bnearer to the active layer is doped in a decreased n-type impurityconcentration, so that a nitride semiconductor device with excellentproperties can be obtained.

The thickness of the first n-type nitride semiconductor layer 5 a isrequired to be thick to a certain extent since the substrate for growingnitride semiconductor, the under layer and the GaN of the hightemperature grown layer are removed and it is preferably 1.5 to 10 μm,more preferably 1 to 5 μm. Especially, since the precision of polishingis about ±0.5 μm, at least 1 μm or more is required. The thickness ofthe second n-type nitride semiconductor layer 5 b is required to beenough to moderate the mechanical impact at the time of removing thesubstrate for growing nitride semiconductor and exposing the firstnitride semiconductor layer and maintain the excellent light emittingproperties of the active layer and it is preferably 0.1 μm or thickerand 1.5 μm or thinner. The second nitride semiconductor layer 5 b alsohas a function of restoring the deterioration of the crystallinityattributed to the doping to the impurity in a high concentration in thefirst nitride semiconductor layer 5 a. The total thickness of the n-typenitride semiconductor layer including the first n-type nitridesemiconductor layer 5 a and the second n-type nitride semiconductorlayer 5 b is preferably thicker than the total thickness of the p-typenitride semiconductor layer and accordingly, the n-electrode is hardlyaffected by heat due to the light emission since the electrode is partedmore from the active layer than the p-electrode formed in the conductivesubstrate side at the viewpoint from the active layer.

(Active Layer)

The active layer 6 to be employed for the invention has a quantum wellstructure comprising at least a well layer of Al_(a)In_(b)Ga_(1-a-b)N,(0≦a≦1, 0≦b≦1, a+b≦1) and a barrier layer of Al_(a)In_(d)Ga_(1-c-d)N,(0≦c≦1, 0≦d≦1, c+d≦1). Further preferably, the foregoing well layer andbarrier layer are Al_(a)In_(b)Ga_(1-a-b)N, (0<a≦1, 0<b≦1, a+b<1) and abarrier layer of Al_(c)In_(d)Ga_(1-c-d)N, (0<c≦1, 0≦d≦1, c+d<1),respectively. The wavelength of the luminescence of the active layer ispreferably 380 nm or shorter and practically the band gap energy of thewell layer is preferably equivalent to the wavelength of 380 nm orshorter.

The nitride semiconductor to be used for the active layer may benon-doped, doped with an n-type impurity, or doped with a p-typeimpurity and preferably a nitride semiconductor which is non-doped orun-doped or doped with an n-type impurity is used to provide a highoutput light emitting device. Further preferably, the well layer is madeto be a un-doped layer and the barrier layer is an n-impurity-dopedlayer to increase the light emitting efficiency of the light emittingdevice.

In this case, the quantum well structure may be either multilayerquantum well structure or a single quantum well structure. Themultilayer quantum well structure is preferable since the output can beimproved and the oscillation threshold value can be decreased.

(Well Layer)

The well layer to be employed for the light emitting device of theinvention may be p-type impurity-doped or n-type impurity-doped orundoped, however it is preferably doped with an n-type impurity becausethe light emitting efficiency can be improved. On the other hand, in thecase of a quaternary mixed crystal of AlInGaN, the crystallinity isdecreased if the impurity concentration is high, and for that, theimpurity concentration is required to be low in order to form a welllayer with excellent crystallinity. Practically, in order to improve thecrystallinity to the maximum extent, an un-doped well layer has to begrown and in this case the well layer with practically free fromimpurities as low impurity concentration as 5×10¹⁶/cm³ or lower ispreferable.

Further, in the case an n-type impurity is doped, doping with an n-typeimpurity concentration in a range of 1×10¹⁸/cm³ or lower and 5×10¹⁶/cm³or higher is preferable. If the n-type impurity concentration is in therange, the carrier concentration can be increased while thecrystallinity deterioration being suppressed and therefore, thethreshold current density and V_(f) can be lowered. Further, in the casethe well layer is doped with an-type impurity, the n-type impurityconcentration of the well layer is preferably adjusted to beapproximately same as or lower than the n-type impurity concentration ofthe barrier layer. Because re-coupling in the well layer is promoted andthe luminescence output can be improved. Further, the well layer and thebarrier layer may be grown while being undoped. In the case of amultilayer quantum well structure, the impurity concentrations of aplurality of well layers are not necessarily same.

In the case of a high output device such as a high output LD and a highpower LED, to be operated with a high quantity of electric current, ifthe well layer is undoped and practically contains no n-type impurity,the re-coupling of carriers in the well layer can be promoted and lightemitting re-coupling can be carried out at a high efficiency. On theother hand, if an n-type impurity is doped into the well layer, anundesired cycle of increase of the carrier density in the well layer,decrease of the probability of the light emitting re-coupling, andincrease of the operation current under constant output is caused toresult in shortening of the device life. Accordingly, it is preferablein the case of a high out put device that the n-type impurity of thewell layer is suppressed to 1×10¹⁸/cm³ or lower, more preferably undopedor practically free from the n-impurity. Consequently, a stably operablelight emitting device with a high output can be obtained.

The well layer constitution illustrated in the following embodiment ofthe invention is preferable for a well layer having a band gap energy tomake luminescence and oscillation possible.

The well layer to be employed for a light emitting device of theinvention is for obtaining luminescence in a wavelength range difficultto obtain by a conventional well layer of InGaN, practicallyluminescence with around 365 nm wavelength, which is the band gap energyof GaN, or with wavelength shorter than that and has band gap energysufficient to give luminance or oscillation with 380 nm or shorterwavelength. In the case of a conventional well layer of InGaN, ifwavelength close to 365 nm wavelength equivalent to the band gap energyof GaN, for example 370 nm wavelength, is tried to obtain, the Incomposition ratio has to be adjusted to be 1% or lower. However, if theIn composition ratio becomes as extremely low as described above, thelight emitting efficiency is decreased to make it impossible to obtain alight emitting device with sufficient output and make it difficult tocontrol the growth. In the invention, a well-layer of a nitridesemiconductor containing Al and In is used, so that the band gap energycan be increased by increasing the Al composition ratio and a lightemitting device provided with a high inner quantum efficiency and lightemitting efficiency even in a wavelength range around 380 nm, in whichefficient light emission has been conventionally difficult, can beobtained by adding In.

The In composition ratio b in the quaternary mixed crystal of InAlGaN tobe used for the well layer is preferably not lower than 0.02 and nothigher than 0.1, more preferably not lower than 0.03 and not higher than0.05. High light emitting efficiency and inner quantum efficiency ascompared with those of the case with the ratio of lower than 0.02 can beobtained by controlling the In composition ratio b to be not lower than0.02 as the lowest limit and the efficiency is further improved bycontrolling the ratio to be not lower than 0.03. On the other hand,deterioration of crystallinity attributed to addition of In can besuppressed by controlling the ratio to be not higher than 0.1 as thehighest limit and the well layer with suppressed crystallinity can beformed by further controlling the ratio to be not higher than 0.05 andtherefore, in the case of forming a plurality of well layers just like acase of a multilayer quantum well structure, the crystallinity of therespective well layers can be make good.

Also, the Al composition ratio a in the quaternary mixed crystal ofInAlGaN to be used for the well layer is preferably not lower than 0.02in the case a band gap energy equivalent to 380 nm wavelength is formed,further preferably not lower than 0.05 in the case a band gap energyequivalent to 365 nm wavelength is formed in order to obtain highluminescence and oscillation.

The thickness of the well layer is preferably not thinner than 1 nm andnot thicker than 30 nm, further preferably not thinner than 2 nm and notthicker than 20 nm, furthermore preferably not thinner than 3.5 nm andnot thicker than 20 nm. Because if it is thinner than 1 nm, the welllayer does not function well and if it is thicker than 30 nm, thecrystallinity of the quaternary mixed crystal of InAlGaN is degraded toresult in deterioration of the properties of the obtained device.Further, if it is not thinner than 2 nm, a layer with relatively evenquality without significant thickness unevenness can be obtained and ifit is not thicker than 20 nm, the crystal growth with suppressed crystaldefect can be made possible. Further, the output can be improved bycontrolling the thickness to be 3.5 nm or thicker. That is, increase ofthe thickness of the well layer promotes luminescent re-coupling of alarge quantity of injected carriers based on the high light emittingefficiency and inner quantum efficiency just like a case of LD operatedwith large electric current and it is especially effective for amultilayer quantum well structure. Further, by controlling the thicknessin a monolayer quantum well structure to be 5 nm or thicker, a similareffect as described above to improve the output can be obtained.

In the case that the In ratio in the InGaN well layer is 0.01 orsmaller, it is preferable that the thickness of the well layer is 10 nmor more and the composition of the barrier layer is Al_(c)Ga_(1-c)N(0<c<1). This provides a light emitting device having high luminescenceefficiency at the wavelength of 370 nm or below.

(Barrier Layer)

In the active layer of the quantum well structure, barrier layers may beformed reciprocally with well layers and a plurality of barrier layersmay be formed to each one well layer. For example, two or more barrierlayers may be sandwiched between neighboring well layers andmultilayered barrier layers and well layers may be reciprocally stacked.

Similar to the well layers, the barrier layers are preferably p-typeimpurity-doped or n-type impurity-doped or undoped, more preferablyn-type impurity-doped or undoped. For example, in the case the barrierlayers are doped with an n-type impurity, the concentration is requiredto be 5×10¹⁶/cm³ or higher. For example, in the case of LED, theconcentration is preferably not lower than 5×10¹⁶/cm³ and not higherthan 2×10¹⁸/cm³. In the case of high output LED or LD, the concentrationis preferably not lower than 5×10¹⁷/cm³ and not higher than 1×10²⁰/cm³,further preferably not lower than 1×10¹⁸/cm³ and not higher than5×10¹⁹/cm³. In this case, the well layers are preferably grown whilebeing doped with practically no n-type impurity or being undoped.

In the case the barrier layers are doped with an n-type impurity, all ofthe barrier layers in the active layer may be doped or some may be dopedand the rest may be undoped. In the case some of the barrier layers aredoped, doping is preferably carried out in the barrier layers which arearranged in the n-type layer side in the active layer. For example,doping is carried out in a barrier layer B_(n) (n denotes a positiveinteger) in the n-th plane from the n-type layer side, so that electronsare injected efficiently into the active layer and an excellent lightemitting device with high light emitting efficiency and inner quantumefficiency can be obtained. Further, also with respect to the welllayers, a similar effect to that of the above mentioned barrier layercase can be obtained by doping the well layer W_(m) (m denotes apositive integer) in the m-th plane from the n-type layer side. Thesimilar effect can be obtained also in the case of doping both barrierlayer and well layer.

The barrier layer constitution illustrated in the following embodimentof the invention is a preferable constitution having a band gap energyto make luminescence and oscillation with wavelength of 380 nm orshorter possible.

With respect to a light emitting device of the invention, it is requiredto use a nitride semiconductor having a higher band gap energy for thebarrier layers than that for the well layers. Especially, in the case ofwell layers with luminescent wavelength in a region of 380 nm orshorter, a quaternary mixed crystal of AlInGaN having a general formulaAl_(c)In_(d)Ga_(1-c-d)N, (0<c≦1, 0≦d≦1, c+d<1) or a ternary mixedcrystal of AlGaN is preferably used for the barrier layers. A quantumwell structure having a high light emitting efficiency as a lightemitting device can be formed by adjusting the Al composition ratio c ina barrier layer to be higher than the Al composition ratio a in a welllayer so as to be c>a and keeping sufficiently high band gap energybetween the barrier layer and the well layer. In the case, a barrierlayer contains In (d>0), the In composition ratio d is not higher than0.1, preferably not higher than 0.05. It is because, in the case, Incomposition ratio becomes higher than 0.1, the reaction of Al and In ispromoted during the growth, the crystallinity could be degenerated, andexcellent film couldn't be formed. By controlling the In compositionratio to be 0.05 or lower, the crystallinity can be further improved andexcellent film can be formed by controlling the In composition ratio dto be 0.05 or lower.

Further, since the difference of band gap energy can be formed mainlydepending on the Al composition ratio and the In composition ratio d ina barrier layer can be optional in a wide range as compared with the Incomposition ratio b in a well layer, it is possible to adjust as d≦b. Insuch as case, since the critical thickness of the well layer and thebarrier layer can be changed, the thickness can be optionally set in thequantum well structure and an active layer with desired properties canbe designed.

The thickness of a barrier layer is similar to that of a well layer andpreferably not thinner than 1 nm and not thicker than 30 nm, morepreferably not thinner than 2 nm and not thicker than 20 nm. Because ifit is thinner than 1 nm, no layer that has even thickness andsufficiently functions as a barrier layer can be formed and if it isthicker than 30 nm, the crystallinity is deteriorated.

(p-Type Clad Layer)

A p-type clad layer is not particularly limited if it has a compositionwith a higher band gap energy than that of the active layer 6 and iscapable of enclosing carriers in the active layer 6, however a layer ofAl_(k)Ga_(1-k)N, (0≦k<1), more preferable Al_(k)Ga_(1-k)N, (0<k<0.4), isemployed. Here, 0.15<k<0.3 is furthermore preferable. Although thethickness of the p-type clad layer is not particularly limited, it ispreferably 0.01 to 0.15 μm, further preferably 0.04 to 0.08 μm. Thep-type impurity concentration of the p-type clad layer is not limited,however it is preferably 1×10¹⁸ to 1×10²¹/cm³, more preferably 1×10¹⁹ to5×10²⁰/cm³. If the p-type impurity concentration is with in theabove-mentioned range, the bulk resistance can be decreased withoutdeteriorating the crystallinity.

The p-type clad layer may be either a monolayer or a multilayerstructure (a superlattice structure). In the case of the multilayerstructure, it may be a multilayer structure composed of the foregoingAl_(k)Ga_(1-k)N and nitride semiconductor layers with a smaller band gapenergy than that of the Al_(k)Ga_(1-k)N. For example, as the layers witha smaller band gap energy, similarly to the case of the n-type cladlayer, In₁Ga_(1-l)N, (0≦1<1), Al_(m)Ga_(1-m)N, (0≦m<1, m>1) can beexemplified. The thickness of each layer composing the multilayerstructure is preferably 100 Å or thinner, more preferably 70 Å, orthinner and furthermore preferably 10 to 40 Å in the case of thesuperlattice structure. In the case the p-type clad layer has themultilayer structure composed of layers with higher band gap energy andlayers with smaller band gap energy, either the layers with higher bandgap energy or the layers with smaller band gap energy may be doped witha p-type impurity. Further in the case both of the layers with higherband gap energy and the layers with smaller band gap energy are doped,the doping amount may be similar or dissimilar.

(p-Type Contact Layer)

The p-type contact layer 8 may be of Al_(f)Ga_(1-f)N, (0≦f<1) andespecially in the case the layer is of Al_(f)Ga_(1-f)N, (0<f<0.3),excellent ohmic contact with an ohmic electrode 9 can be obtained. Thep-type impurity concentration is preferably 1×10¹⁷/cm³ or higher.

Further, the p-type contact layer 8 preferably has a composition gradedin a manner that the p-type impurity concentration is high in theconductive substrate side and the mixed crystal ratio of Al becomeslower. In this case the graded composition may be a compositionsuccessively changed or a composition changed intermittently step bystep. For example, the p-type contact layer 8 may be composed of a firstp-type contact layer contacting with the ohmic electrode 9 and having ahigh p-type impurity concentration and a low Al composition ratio and asecond p-type contact layer having low p-type impurity concentration anda high Al composition ratio. Good ohmic contact can be obtained by thefirst p-type contact layer and self-absorption can be prevented by thesecond p-type contact layer.

The composition of the first p-type contact layer is preferablyAl_(g)Ga_(1-g)N, (0≦g<0.05), more preferably 0<g<0.01. If the Alcomposition ratio is within the above-mentioned range, even if highconcentration doping with p-type impurity is carried out, inactivationof the impurity can be prevented and excellent ohmic contact can beobtained. The p-type impurity concentration of the first p-type contactlayer is preferably 1×10¹⁹/cm³ to 1×10²²/cm³, further preferably5×10²⁰/cm³ to 5×10²¹/cm³. Further, the thickness of the first p-typecontact layer is preferably 100 to 500 Å, more preferably 150 to 300 Å.

On the other hand, the composition of the second p-type contact layer ispreferably Al_(h)Ga_(1-h)N, (0≦h<0.1), more preferably 0.1<h<0.05. Ifthe Al composition ratio is within the above-mentioned range,self-absorption can be prevented. The p-type impurity concentration ofthe second p-type contact layer is preferably 1×10²⁰/cm³ or lower,further preferably 5×10¹⁸/cm³ to 5×10¹⁹/cm³. Further, the thickness ofthe second p-type contact layer is preferably 400 to 1,200 Å, morepreferably 800 to 1,200 Å.

(Phosphor)

(Types of Phosphors)

In a nitride semiconductor device, particularly a light emitting device,of the invention, light rays with a variety of wavelength values can beemitted by forming a coating layer or a sealing member containing aphosphor substance that emits luminescence with different wavelengthvalues by absorbing a portion or entire luminescence from the activelayer in the nitride semiconductor device composed by bonding to asupporting substrate. Some examples of the phosphor substance are asfollows. As green-emitting phosphors, SrAl₂O₄:Eu; Y₂SiO₅:Ce,Tb;MgAl₁₁O₁₉:Ce,Tb; Sr₇Al₁₂O₂₅:Eu; (at least one or more of Mg, Ca, andBa)Ga₂S₄:Eu; BaAl₁₂O₁₉:Eu, Mn; ZnS:Cu,Al can be exemplified(hereinafter, described as “group 1”). Also, as blue-emitting phosphors,Sr₅(PO₄)₃Cl:Eu; (SrCaBa)₅(PO₄)₃Cl:Eu; (BaCa)₅(PO₄)₃Cl:Eu; (at least oneor more of Mg, Ca (PO₄)₆Cl₂:Eu, Mn; BaAl₁₂O₁₉:Mn; Ca(PO₄)₃Cl:Eu;CaB₅O₉Cl:Eu can be exemplified (hereinafter, described as “group 2”). Asred-emitting phosphors, Y₂O₂S:Eu; La₂O₂S:Eu; and Gd₂O₂S:Eu can beexemplified. Further, as yellow-emitting phosphors, YAG; Tb₃Al₅O₁₂:Ce;(BaSrCa)₂SiO₄:Eu; CaGaS₄:Eu is exemplified (hereinafter, described as“group 3”). As blue-gree-emitting phosphors, Sr₄Al₁₄O₂₅:Eu isexemplified. As yellow-red-emitting phosphors, Ca₂Si₅N₈:Eu isexemplified, and as orange-emitting phosphors, ZnS:Mn is exemplified,respectively,

Especially, by adding YAG and a phosphor selected from group 1, anemission of white light is made possible and applications of the deviceis significantly wider, for example, to light sources. YAG is(Y_(1-x)Gd_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:R(R denotes one or more elementsselected from Ce, Tb, Pr, Sm, Eu, Dy, and Ho; 0<R<0.5). Practicalexamples are (Y_(0.8)Gd_(0.2))₃ Al₅O₁₂:Ce andY₃(Al_(0.8)Ga_(0.2))₅O₁₂:Ce.

With respect to the materials which absorb a portion or entire lightrays and emit luminescence with different wavelength values, those whichabsorb visible light rays and emit luminescence with differentwavelength values are limited and selectivity of such materials is poor.However, materials which absorb UV rays and emit luminescence withdifferent wavelength values exist considerably many and the materialscan be selected depending on a variety of uses. One reason for theselectivity of the materials is because phosphors which absorb UV rayshave high light conversion efficiency as compared with that in the caseof visible light rays.

Especially, in the case of white light, high color-rendering white lightcan be obtained and thus the application possibility is further widened.White light is preferably obtained by combining a plurality ofphosphors, for example, three kinds of phosphors selected from group 1,group 2 and group 3, respectively; two kinds selected from group 2 andgroup 4, respectively; two kinds selected from a group of blue-greenphosphors and group 3, respectively. By applying the present inventionto the nitride semiconductor device emitting UV light and coating thedevice with phosphor materials, a white light emitting device with anextremely high light-converting efficiency is provided.

[Particle Size of Phosphor]

The particle size of a phosphor to be employed in the invention ispreferably in a range of 6 to 50 μm, more preferably 15 to 30 μm, formean particle size. Phosphors having such a particle size has a highluminescence absorption property and high conversion efficiency and awide range of excitation wavelength. Phosphors having a particle sizesmaller than 6 μm are relatively easy to agglomerate one another and bedensified and precipitated in a liquid state resin and thereforedecrease the light transmissivity and moreover, they have a lowluminescence absorption property and low conversion efficiency and anarrow range of excitation wavelength.

In this specification of the invention, the particle size of a phosphoris a value obtained by particle size distribution curve on the basis ofvolume and the particle size distribution curve on the basis of volumecan be obtained by measuring the particle size distribution of phosphorsby laser diffraction and diffusion method. Practically, under conditionsof 25° C. temperature and 70% humidity, a phosphor is dissolved in anaqueous solution of 0.05% concentration of sodium hexametaphosphate andsubjected to the measurement by a laser diffraction type particle sizemeasuring apparatus (SALD-2000A) in a particle size range of 0.03 μm to700 μm. Also, in this specification, the center particle size of aphosphor is a particle size value calculated as the 50% integrated valuein the particle size distribution curve on the basis of the volume. Itis preferable that many phosphor particles having the center particlesize value are added and the frequency ratio of the phosphor particlesis preferably 20 to 50%. Use of such phosphor particles in a smalldispersion of the particle size gives a light emitting device withsuppressed color unevenness and good contrast.

(Yttrium Aluminum Oxide Type Phosphors)

Among phosphors to be employed for the invention are preferablephosphors (YAG type phosphors) mainly consisting of yttrium aluminumoxide type phosphors which are excited by luminescence emitted out of asemiconductor light emitting device comprising a light emitting layer ofa nitride semiconductor and emit luminescence and which are activated bycerium (Ce) or praseodymium (Pr) since they are capable of emittingwhite light rays. Practical examples of the yttrium aluminum oxide typephosphors are YAlO₃:Ce, Y₃Al₅O₁₂:Ce (YAG:Ce), Y₄Al₂O₉:Ce and theirmixtures. The yttrium aluminum oxide type phosphors may contain at leastone of Ba, Sr, Mg, Ca and Zn. Addition of Si can suppress reaction ofcrystal growth and makes the particle size of phosphor particles even.

In this specification, a yttrium aluminum oxide type phosphor excited byCe is especially broadly defined and include phosphors in which yttriumis partially or entirely replaced with at least one element selected agroup consisting of Lu, Sc, La, Gd and Sm and/or aluminum is partiallyor entirely replaced with Ba, TI, Ga, In and which have luminescenceemitting function.

Further in details, the yttrium aluminum oxide type phosphor meansphotoluminescent phosphors defined by a general formula(Y_(z)Gd_(1-z))₃Al₅O₁₂:Ce (0<z≦1) and a general formula(Re_(1-a)Sm_(a))₃Re'₅O₁₂:Ce (0≦a<1; 0≦b≦1; Re denotes at least oneelement selected from Y, Gd, La, and Sc; and Re' denotes at least oneelement selected from Al, Ga and In). Since the phosphors have garnetstructure, they are highly durable to heat, light and water and capableof giving an excitation spectrum peak close to 450 nm. Also, they haveluminescence peak near 580 nm and have a broad luminescence spectrahaving tails to 700 nm.

The photoluminescent phosphors can be provided with a high excitationluminescence efficiency in a long wavelength range of 460 nm or longerby adding Gd (gadolinium) in the crystal. The luminescent peakwavelength is shifted to the longer wavelength side and the entireluminescence wavelength is also shifted to the longer wavelength side byincreasing the content of Gd. That is, in the case intense red-emittingcolor is required, it can be accomplished by increasing the amount ofreplacement with Gd. On the other hand, along with increase of Gd, theluminous brightness of the photoluminescence attributed to the bluecolor light tends to be decreased. Further, depending on the necessity,Tb, Cu, Ag, Au, Fe, Cr, Nd, Dy, Co, Ni, Ti, and Eu may be contained inaddition to Ce. Moreover, in the composition of the yttrium aluminumgarnet-based phosphor having the garnet structure, replacement of aportion of Al with Ga can shift the luminescence wavelength to theshorter wavelength side and replacement of a portion of Y with Gd canshift the luminescence wavelength to the higher wavelength side.

In the case a portion of Y is replaced with Gd, it is preferable tosuppress the replacement with Gd to less than 10% and to control thecontent of Ce (replacement ratio) to be 0.03 to 1.0. If replacement withGd is less than 20%, the green color component is increased and the redcolor component is decreased, however it is made possible to catch thered color component and obtain desired color tone without decreasingbrightness by increasing the Ce content. With such a composition, thetemperature properties can be improved and the reliability of a lightemitting diode can be improved. Further, use of a photoluminescentphosphor adjusted so as to contain a large quantity of the red colorcomponent makes it possible to fabricate a light emitting apparatuscapable of emitting neutral tints such as pink and the like.

Such photoluminescent phosphors can be obtained in the following manner.As raw materials of Y, Gd, Al, Ce, and Pr, oxides or compounds easy tobe oxides at a high temperature are used and they are sufficiently mixedin stoichiometric ratios to obtain raw materials. Alternatively, a rawmaterial mixture is obtained by mixing solutions produced by dissolvingrare earth elements of Y, Gd, Ce, and Pr in acids, producingcoprecipitates by adding oxalic acid to the mixed solution, obtainingoxides of the coprecipitates by firing the coprecipitates and mixing theobtained oxides with aluminum oxide. A proper amount of a fluoride suchas barium fluoride, ammonium fluoride or the like as a flux to theobtained raw material mixture and the resulting mixture is packed in acrucible and fired at 1,350 to 1,450° C. for 2 to 5 hours in air toobtain a fired product and the fired product is ball-milled in water,washed, separated, dried, and finally sieved to obtain the phosphors.

In the nitride semiconductor device of the invention, suchphotoluminescent phosphors may be mixtures of two or more ofcerium-activated yttrium aluminum garnet type phosphors and otherphosphors. Light rays with desired color tone can be easily accomplishedby mixing two types of yttrium aluminum garnet type phosphors withdifferent replacement amounts of Y with Gd. Especially, a phosphor witha higher replacement amount is used as the foregoing phosphor and aphosphor with a less replacement amount of without replacement is usedas the forgoing phosphor with a middle particle size, so that both ofthe color rendering property and the brightness can be simultaneouslyimproved.

(Nitride Type Phosphors)

The phosphors to be used in the invention may include nitride typephosphors containing N, at least one element selected from Be, Mg, Ca,Sr, Ba, and Zn, and at least one element selected from C, Si, Ge, Sn,Ti, Zr, and Hf, and activated by at least one element selected from rareearth elements. The nitride-based phosphors can be excited by absorptionof visible light rays and UV rays emitted out of a light emittingdevice, or luminescence from a YAG-type phosphor and emit luminescence.Among the nitride phosphors are preferable Mn-containing siliconnitrides such as Sr—Ca—Si—N:Eu, Ca—Si—N:Eu, Sr—Si—N:Eu, Sr—Ca—Si—O—N:Eu,Ca—Si—O—N:Eu, and Sr—Si—O—N:Eu.

The basic elements constituting of the phosphors can be defined by ageneral formula L_(x)Si_(y)N_((2/3x+4/3y)):Eu orL_(x)Si_(y)O_(z)N_((2/3x+4/3y−2/3z)):Eu (L denotes Sr, Ca, or Sr andCa). In the general formula, X and Y are preferably as follows; X=2, Y=5or X=1, Y=7, however optional ones are also usable. Practically, thebasic constitutions of the phosphors are preferably Mn-added(Sr_(x)Ca_(1-x))₂Si₅N₈:Eu, Sr₂Si₅N₈:Eu, Ca₂Si₅N₈:Eu,Sr_(x)Ca_(1-x)Si₇N₁₀:Eu, SrSi₇N₁₀:Eu, and CaSi₇N₁₀:Eu. Thesecompositions of the phosphors may contain at least one or more elementsselected from Mg, Sr, Ca, Ba, Zn, B, Al, Cu, Mn, Cr, and Ni. Further,mixing ratio of Sr and Ca may be changed based on the necessity. Use ofSi for the compositions of the phosphors provides economical andexcellently crystalline phosphors.

With respect to the nitride type phosphors, europium (Eu), a rare earthelement, is preferably used as the luminescence center. Europium hasmainly divalent and trivalent energy levels. The phosphors of theinvention contain Eu²⁺ as an activator for the mother substances,alkaline earth nitride silcides. Eu²⁺ is easy to be oxidized and Eu iscommercialized in form of trivalent Eu₂O₃ composition. However, Ogreatly affects in the case of the commercialized Eu₂O₃ to make itdifficult to obtain good phosphors. Therefore, compounds obtained byremoving O from Eu₂O₃ to the outside of the system are preferable to beused. For example, simple substance europium and europium nitride arepreferable. However, that is not so in the case Mn is added.

Mn, an additive, promotes diffusion of Eu²⁺ and improves theluminescence brightness, the energy efficiency, and the luminescenceefficiency such as quantum efficiency or the like. Mn may be added toraw materials or added in form of Mn simple substance or a Mn compoundduring the production process and fired together with the raw materials.However, Mn is not contained in the basic constitutions after the firingor remains in a slight amount as compared with the initial additionamount. It is supposedly attributed to that Mn is scattered.

The nitride type phosphors preferably contain at least one elementsselected from a group consisting of Mg, Sr, Ca, Ba, Zn, B, Al, Cu, Mn,Cr, O and Ni in the basic constitutions or together with the basicconstitutional elements. These elements have functions of enlarging theparticle size or increasing the luminescence brightness. Further, B, Al,Mg, Cr, and Ni have functions of suppressing afterglow.

Such nitride type phosphors emit light rays in a region from yellow tored by absorbing a portion of blue color light emitted by a lightemitting device. A light emitting device emitting warm color type whitelight by mixing yellow to red color light rays emitted from the nitridetype phosphors with blue color light emitted from a light emittingdevice can be obtained by using the nitride type phosphors together withYAG type phosphors for the light emitting apparatus having theabove-mentioned constitution. The phosphors to be added besides thenitride type phosphors preferably include cerium-activated yttriumaluminum oxide phosphors because the desired chromaticity can beadjusted by adding the foregoing yttrium aluminum oxide phosphors. Thecerium-activated yttrium aluminum oxide phosphors absorb a portion ofblue color light emitted from a light emitting device and emit light ina yellow region. The blue color light emitted from a light emittingdevice and the yellow color light of the yttrium aluminum oxidephosphors are mixed to emit bluish white light. Accordingly, a lightemitting apparatus capable of emitting white type mixed color light bymixing the yttrium aluminum oxide phosphors and red-emitting phosphorstogether in a color conversion layer and combining the blue color lightemitted from a light emitting device all together. Particularlypreferable apparatus is a white light emitting apparatus having thechromaticity at the Planck Ian locus in the chromaticity diagram.Incidentally, in order to obtain a light emitting apparatus with adesired color temperature, the amount of the yttrium aluminum oxidephosphors and the amount of the red-emitting phosphors can be properlychanged. The light emitting apparatus capable of emitting white typemixed color light is provided with improved CIE 1974 special colorrendering index R9. A light emitting apparatus which emits white lightonly based on the combination of a conventional light emitting devicefor emitting blue color light and cerium-activated yttrium aluminumoxide phosphors has CIE 1974 special color rendering index R9approximately zero around the color temperature Tcp=4,600 K and isinsufficient in the red-emitting components. Therefore, improvement ofthe CIE 1974 special color rendering index R9 has been a matter to besolved and use of the red-emitting phosphors together with the yttriumaluminum oxide phosphors can increase the CIE 1974 special colorrendering index R9 around the color temperature Tcp=4,600 K to about 40.

Next, a production method of a phosphor ((Sr_(x)Ca_(1-x))₂Si₅N₈:Eu)according to the invention will be described, however the description isillustrative and is not to be construed as limiting the productionmethod. The above-mentioned phosphor contains Mn and O.

(1) At first, raw materials of Sr and Ca are crushed. Simple substancesare preferable to be used as the raw materials of Sr and Ca, howevercompounds such as imides and amides may be used. Further, the rawmaterials of Sr and Ca may be those containing B, Al, Cu, Mg, Mn, andAl₂O₃. The raw materials of Sr and Ca are crushed in a globe box inargon atmosphere. Sr and Ca obtained by crushing preferably have anaverage particle size of about 0.1 to 15 μm, but the average particlesize is not limited to the range. The purity of Sr and Ca is preferably2N or higher, but it is not limited. In order to improve the mixingstate, at least one of metal Ca, metal Sr, and metal Eu may be alloyedand then nitrided and crushed to use the resulting powder as a rawmaterial.(2) A raw material of Si is crushed. A simple substance is preferable tobe used, however compounds such as nitrides, imides and amides may beused. For example, Si₃N₄, Si(NH₂)₂, and Mg₂Si can be used. The purity ofSi is preferably 3N or higher, however it may contain compounds such asAl₂O₃, Mg, metal borides (CO₃B, Ni₃B, CrB), manganese oxide, H₃BO₃,B₂O₃, Cu₂O, CuO and the like but it is not limited. Si, as same as theraw materials of Sr and Ca, is crushed in a globe box in argonatmosphere or nitrogen atmosphere. The average particle size of the Sicompound is preferably about 0.1 to 15 μm.(3) Next, the raw materials of Sr and Ca are nitrided in nitrogenatmosphere. The reaction formula is shown as the following formula 1 andformula 2.3Sr+ N₂→Sr₃N₂  (formula 1)3Ca+ N₂→Ca₃N₂  (formula 2)

Sr and Ca are nitrided at 600 to 900° C. for about 5 hours in nitrogenatmosphere. Sr and Ca may be mixed and nitrided or independentlynitrided. Accordingly, Sr and Ca nitrides can be obtained. The Sr and Canitrides are preferable to have a high purity, however thosecommercialized can be used.

(4) The raw material of Si is nitrided in nitrogen atmosphere. Thereaction formula is shown as the following formula 3.3Si+2N₂→Si₃N₄  (formula 3)

Si is also nitrided at 800 to 1,200° C. for about 5 hours in nitrogenatmosphere. Accordingly, Si nitride can be obtained. The Si nitride tobe used for the invention is preferable to have a high purity, howeverthose commercialized can be used.

(5) Nitrides of Sr, Ca, or Sr—Ca are crushed. Nitrides of Sr, Ca, orSr—Ca are crushed are crushed in a globe box in argon atmosphere ornitrogen atmosphere. Similarly, the Si nitride is also crushed. Further,an Eu compound, Eu₂O₃, is also similarly crushed. As an Eu compound,europium oxide is used, however metal europium, europium nitride and thelike can be used. Besides, as the raw materials of Eu, imides and amidesmay be used. Although high purity europium oxide is preferable,commercialized ones can be used. The average particle size of the alkaliearth nitride, silicon nitride, and europium oxide after crushing ispreferably about 0.1 to 15 μm.

The foregoing raw materials may contain at least one element selectedfrom a group consisting of Mg, Sr, Ca, Ba, Zn, B, Al, Cu, Mn, Cr, O andNi. Further, the above-mentioned elements such as Mg, Zn, B and so onmay be mixed while the mixing amount being properly adjusted during thefollowing mixing steps. These elements may be added solely to the rawmaterials, however in general, they are added in form of compounds. Suchkind compounds include H₃BO₃, Cu₂O₃, MgCl₂, MgO.CaO, Al₂O₃, metalborides (CrB, Mg₃B₂, AlB₂, MnB), B₂O₃, Cu₂O, CuO and the like.

(6) After the above-mentioned crushing is carried out, nitrides of Sr,Ca, Sr—Ca, Si nitride, and an Eu compound Eu₂O₃ are mixed and Mn isadded. Since the mixture of them is easy to be oxidized, mixing iscarried out in a globe box in Ar atmosphere or nitrogen atmosphere.(7) Finally, the mixture of nitrides of Sr, Ca, Sr—Ca, Si nitride, andan Eu compound Eu₂O₃ is fired in ammonia atmosphere. A phosphor definedas (Sr_(x)Ca_(1-x))₂Si₅N₈:Eu containing Mn can be obtained by firing.However, the composition of an aimed phosphor can be changed by changingthe mixing ratios of the respective raw materials. Firing can be carriedout by using a tubular furnace, a small size furnace, a high frequencyfurnace, a metal furnace and the like. The firing temperature may be ina range of 1,200 to 1,700° C., preferably 1,400 to 1,700° C. Firing iscarried out preferably by one-step firing method involving graduallyincreasing the temperature and heating at 1,200 to 1,500° C. for severalhours, however it may be carried out by two-step firing (or multi-stepfiring) method involving first-step firing at 800 to 1,000° C.,gradually increasing the temperature and second-step firing at 1,200 to1,500° C. Raw materials of the phosphors are preferably fired using acrucible or a boat made of boron nitride (BN)-based materials. Besidesthe crucible made of boron nitride-based materials, a crucible made ofalumina (Al₂O₃)-based materials can be used.

By the above-mentioned production method, an aimed phosphor can beobtained.

In an embodiment of the invention, as a phosphor emitting luminescencebearing red color, particularly nitride type phosphors are used, howeverin the invention, a light emitting apparatus may comprise theabove-mentioned YAG type phosphor and a phosphor capable of emitting redtype luminescence. Such a phosphor capable of emitting red typeluminescence is a phosphor excited by light rays with wavelength of 400to 600 nm and emits luminescence and, for example, Y₂O₂S:Eu; La₂O₂S:Eu;CaS:Eu; SrS:Eu; ZnS:Mn; ZnCdS:Ag,Al; and ZnCdS:Cu, Al and the like canbe exemplified. The color rendering properties of the light emittingapparatus can be improved by using the phosphor capable of emitting redtype luminescence together with the YAG type phosphor.

Two or more kinds of YAG type phosphors and phosphor capable of emittingred type luminescence such as nitride type phosphors produced in theabove-mentioned manner may exist in a color conversion layer composed ofa single layer in the side end face of a light emitting device or one ormore kinds of them may exist respectively in each layer of a colorconversion layer composed of two layer. With such a constitution, mixedcolor light by mixing colors of luminescence from different types ofphosphors can be obtained. In this case, in order to well mix the colorof luminescence emitted from each phosphor and suppress the colorunevenness, the average particle sizes and the shapes of the respectivephosphors are preferably identical. In consideration of absorption of aportion of luminescence with wavelength converted by the YAG phosphors,the nitride type phosphors are preferable to form a color conversionlayer so as to arrange the nitride type phosphors nearer to the side endface of the light emitting device than the YAG type phosphors. With sucha constitution, the nitride type phosphors can be prevented fromabsorbing a portion of luminescence, whose wavelength is converted bythe YAG phosphors and the color rendering properties of the mixedluminescence from both types of phosphors can be improved as comparedwith those in the case where the YAG type phosphors and nitride typephosphors are contained in form of a mixture.

(Package)

The semiconductor light emitting device obtained by the invention can bemounted, for example, in the following package to give a light emittingdevice. At first, as illustrated in FIGS. 6A to 6C, a nitridesemiconductor light emitting device 30 is disposed in a heat sink 32provided with a lead frame 34 and a conductive wire 36 is bonded to thelead frame 34 from the semiconductor light emitting device 30. Afterthat, transparent glass 38 is used for packaging to obtain a lightemitting device.

Further, a package illustrated in FIGS. 7A to 7C, in place of FIGS. 6Ato 6C, may be employed. FIG. 7A illustrates a light emitting deviceproduced by the following. A packaging resin 40 having a heat sink 42 ismade ready, a semiconductor light emitting device 30 is installed in aheat sink 42, and a conductive wire 46 is bonded to the lead frame 44from the semiconductor light emitting device 30. After that, pottingresin 48 of such as silicone is applied to the foregoing semiconductorlight emitting device 30. Further, a lens 49 is formed thereon to obtaina light emitting device. The light emitting devices illustrated in FIG.6 and FIG. 7 are preferably provided with a protection apparatus 31 toprotect the semiconductor light emitting devices 30 from staticelectricity.

Incidentally, although the above-mentioned embodiments are describedalong with the cases that the invention is applied to the light emittingdiodes for UV emitting grown on sapphire substrates, the invention isnot limited to these embodiments. It can be applied to the cases wheredevices are grown on substrates for growing nitride semiconductor otherthan sapphire and diodes emitting blue color light rays with wavelengthother than UV rays. Further, the invention can be applied to not onlythe light emitting diodes but also laser diodes.

EXAMPLES Example 1

In this Example, the invention was applied to a light emitting diodewith 375 nm wavelength and a nitride semiconductor device was producedaccording to a fabrication method of the invention as illustrated inFIGS. 1A to 1E.

(Substrate for Growing Nitride Semiconductor)

A substrate of sapphire (C-plane) was used as a substrate for growingnitride semiconductor 1 and surface cleaning was carried out at 1,050°C. in hydrogen atmosphere in a MOCVD reaction vessel.

(Under Layer 2)

Buffer layer 3: Successively, a buffer layer 3 of GaN in a thickness ofabout 200 Å was grown on the substrate at 510° C. in hydrogen atmosphereusing ammonia and TMG (trimethylgallium).

(High Temperature Grown Layer 4)

High temperature grown layer 4: After growth of the buffer layer 3, ahigh temperature grown nitride semiconductor 4 of undoped GaN in 5 μmthickness was grown on the substrate by stopping only TMG supply,increasing the temperature to 1,050° C., and using TMG and ammonia asraw material gases when it reached at 1,050° C.

(n-Type Clad Layer 5)

Next, an n-type clad layer 5 of n-type Al_(0.18)Ga_(0.82)N doped with Siin a concentration of 5×10¹⁷/cm³ and with a thickness of 400 Å was grownby using TMG, TMA, ammonia, and silane at 1,050° C.

(Active Layer 6)

Next, barrier layers of Si-doped Al_(0.1)Ga_(0.9)N and well layers ofundoped In_(0.03)Al_(0.02)Ga_(0.95)N were laminated in the order ofbarrier layer (1)/well layer (1)/barrier layer (2)/well layer(2)/barrier layer (3)/well layer (3) by controlling the temperature at800° C. and using TMI (trimethylindium), TMG, TMA as raw material gases.In this case, the thickness was controlled to be 200 Å for the barrierlayer (1), 40 Å for the barrier layers (2) and (3), and 70 Å for thewell layers (1) and (2). The active layer 6 had the total thicknessabout 420 Å and a multilayer quantum well (MQW) structure.

(p-Type Clad Layer 7)

Next, a p-type clad layer 7 of p-type Al_(0.2)Ga_(0.8)N doped with Mg ina concentration of 1×10²⁰/cm³ and with a thickness of 600 Å was grown byusing TMG, TMA, ammonia, and Cp₂Mg (cyclopentadienylmagnesium) at 1,050°C. in hydrogen atmosphere.

(p-Type Contact Layer 8)

Successively, a first p-type contact layer of p-type Al_(0.04)Ga_(0.96)Ndoped with Mg in a concentration of 1×10¹⁹/cm³ and with a thickness of0.1 μm was grown on the p-type clad layer by using TMG, TMA, ammonia,and Cp₂Mg and after that, a second p-type contact layer of p-typeAl_(0.01)Ga_(0.99)N doped with Mg in a concentration of 2×10²¹/cm³ andwith a thickness of 0.02 μm was grown by adjusting the gas flow rates.

On completion of the growth, the wafer was annealed at 700° C. in thereaction vessel to further lower the resistance of the p-type layers 7and 8.

(First Bonding Layer 9)

After annealing, the wafer was taken out of the reaction vessel and a Rhfilm in a thickness of 2,000 Å was grown on the p-type contact layer toform a p-electrode. After that, ohmic annealing was carried out at 600°C. and then an insulating protection film SiO₂ in a thickness of 0.3 μmwas formed on the exposed face other than the p-electrode.

Next, a multilayer film of Ni—Pt—Au—Sn—Au with a thickness of 2,000Å–3,000 Å–3,000 Å–30,000 Å–1,000 Å was formed on the p-electrode. Inthis case, Ni was a bonding layer, Pt was a barrier layer, Sn was afirst eutectic forming layer, Au between the Pt and Sn was a layer forpreventing diffusion of Sn to the barrier layer, and the outermost Aulayer was a layer for improving adhesion strength to a second eutecticforming layer.

(Second Bonding Layer 11)

On the other hand, a metal substrate 10 with a thickness of 200 μm andmade of a composite consisting of Cu 30% and W 70% was used as aconductive supporting substrate and a closely sticking layer of Ti, abarrier layer of Pt and a second eutectic forming layer of Au withthickness of 2,000 Å–3,000 Å–12,000 Å were formed in this order on thesurface of the metal substrate 10.

Next, while the first bonding layer 9 and the second bonding layer 11being set face to face, the bonding laminate and the conductivesupporting substrate 10 were bound by heating and pressing at 250° C.heater temperature. Accordingly, the metals of the first eutecticforming layer and the second eutectic forming layer were mutuallydiffused to form an eutectic.

(Removal of Substrate for Growing Nitride Semiconductor 1)

Next, after the sapphire substrate 1 was removed from the laminate forbonding to which the conductive supporting substrate 10 was bound, theexposed buffer layer 3 and the high temperature grown layer 4 werepolished and polishing was carried out until the AlGaN layer of then-type clad layer 5 was exposed to eliminate surface roughness.

(n-Electrode)

Next, a multilayer electrode of Ti—Al—Ti—Pt—Au in a thickness of 100Å–2,500 Å–1,000 Å–2,000 Å–6,000 Å was formed on the n-type clad layer 5functioning also as the n-type contact to form an n-electrode. Afterthat, the conductive supporting substrate 10 was polished to a thicknessof 100 μm and then as a pad electrode 13 for the p-electrode, amultilayer film of Ti—Pt—Au in a thickness of 1,000 Å–2,000 Å–3,000 Åwas formed on the rear face of the conductive supporting substrate 10.Since the p-pad electrode 13 serves as a bonding portion to a package ofthe device, a material suitable for bonding to the package is preferablyselected for the p-pad electrode. Finally, devices were separated bydicing.

The obtained LED was in size of 1 mm×1 mm, emitted UV luminescence with373 nm wavelength at 20 mA in forward direction and had output power of4.2 mW and Vf of 3.47 V.

Example 2

An LED obtained in the same manner under same conditions as those ofExample 1 except that the p-type electrode in the first bonding layerwith a thickness of 2,000 Å was formed using Ag had output power of 5.8mW and Vf of 4.2V.

Example 3

This Example was carried out under same conditions as those of Example 1except that the laser radiation method was employed in place of thepolishing method at the time of removing the substrate for growingnitride semiconductor 1.

(Removal of Substrate for Growing Nitride Semiconductor 1)

With respect to the laminate for bonding to which the conductivesupporting substrate 10 was bound, laser beam in linear state of 1 mm×50mm was radiated with an output power of 600 J/cm² to the entire surfaceof the opposed face of the sapphire substrate 1 from the under layerside by scanning using KrF excimer laser with wavelength of 248 nm. Thenitride semiconductor of the under layer 2 was decomposed by the laserradiation to remove the sapphire substrate 1.

The obtained LED had luminescence peak wavelength of 373 nm at 20 mAelectric power in forward direction and had Vf of 3.47 V and outputpower of 4.2 mW. Further, as compared with the case of Example 1, sincethe sapphire substrate did not required to be polished, the time takenfor the fabrication could be remarkably shortened. The light emittingoutput was considerably improved as compared with a conventional device.

Example 4

A nitride semiconductor device was fabricated under the same conditionsas those of Example 3. Further, a coating layer of SiO₂ containing YAGphosphor and a blue-emitting phosphor selected from group 2 was formedon the entire surface of the nitride semiconductor device.

Accordingly, a nitride semiconductor light emitting device capable ofemitting high color-rendering white luminescence and having littleself-absorption and high conversion efficiency was obtained.

Example 5

A nitride semiconductor device was fabricated under the same conditionsas those of Example 3 and in this Example, a plurality of nitridesemiconductor devices were formed in dot-like arrangement on aconductive substrate. A plurality of the nitride semiconductor deviceswere packaged while exposed face being formed in some portions. Further,a coating layer of SiO₂ containing YAG phosphor and a blue-emittingphosphor selected from group 2 was formed on the exposed face.

Accordingly, a nitride semiconductor light emitting apparatus comprisinga plurality of arranged nitride semiconductor devices capable ofemitting white luminescence, having a large surface area, and capable ofemitting white luminescence was obtained. The apparatus was usable as alight source for luminescence.

Example 6

In this Example, the invention was applied to a light emitting diodewith 365 nm wavelength and a nitride semiconductor device illustrated inFIGS. 5A and 5B was produced according to a fabrication method of theinvention as illustrated in FIG. 1A to 1E.

(Substrate for Growing Nitride Semiconductor)

A substrate of sapphire (C-plane) was used as a substrate for growingnitride semiconductor 1 and surface cleaning was carried out at 1,050°C. in hydrogen atmosphere in a MOCVD reaction vessel.

(Under Layer 2)

Buffer layer 3: Successively, a buffer layer 2 of GaN in a thickness ofabout 200 Å was grown on the substrate at 510° C. in hydrogen atmosphereusing ammonia and TMG (trimethylgallium).

(High Temperature Grown Layer 4)

High temperature grown layer 4: After growth of the buffer layer, a hightemperature grown nitride semiconductor of undoped GaN in 5 μm thicknesswas grown by stopping only TMG supply, increasing the temperature to1,050° C., and using TMG and ammonia as raw material gases when itreached at 1,050° C.

(n-Type Clad Layer 5)

Next, an n-type clad layer 5 of n-type Al_(0.08)Ga_(0.92) N doped withSi in a concentration of 1×10¹⁹/cm³ and with a thickness of 2.5 μm wasgrown by using TMG, TMA, ammonia, and silane at 1,050° C.

(Active Layer 6)

Next, barrier layers of Al_(0.08)Ga_(0.92)N doped with Si in aconcentration of 1×10¹⁹/cm³ and well layers of undoped In_(0.1)Ga_(0.9)Nwere laminated in the order of barrier layer (1)/well layer (1)/barrierlayer (2)/well layer (2)/barrier layer (3)/well layer (3)/barrier layer(4) by controlling the temperature at 900° C. and using TMI(trimethylindium), TMG, TMA as raw material gases. In this case, thethickness was controlled to be 370 Å for the barrier layers (1), (2),(3) and (4) and 80 Å for the well layers (1), (2), and (3). Only thebarrier layer (4) was undoped. The active layer 6 had the totalthickness about 1,700 Å and a multilayer quantum well structure.

(p-Type Clad Layer 7)

Next, a p-type clad layer 7 of Al_(0.2)Ga_(0.8)N doped with Mg in aconcentration of 1×10²⁰/cm³ and with a thickness of 370 Å was grown byusing TMG, TMA, ammonia, and Cp₂Mg (cyclopentadienylmagnesium) at 1,050°C. in hydrogen atmosphere.

(p-Type Contact Layer 8)

Successively, a first p-type contact layer of Al_(0.07)Ga_(0.93)N dopedwith Mg in a concentration of 1×10¹⁹/cm³ and with a thickness of 0.1 μmwas grown on the p-type clad layer by using TMG, TMA, ammonia, and Cp₂Mgand after that, a second p-type contact layer of Al_(0.07)Ga_(0.93)Ndoped with Mg in a concentration of 2×10²¹/cm³ and with a thickness of0.02 μm was grown by adjusting the gas flow rates.

On completion of the growth, in nitrogen atmosphere, the wafer wasannealed at 700° C. in the reaction vessel to further lower theresistance of the p-type layers 7 and 8.

(First Bonding Layer 9)

After annealing, the wafer was taken out of the reaction vessel and a Rhfilm in a thickness of 2,000 Å was grown on the p-type contact layer 8to form a p-electrode. After that, ohmic annealing was carried out at600° C. and then an insulating protection film SiO₂ in a thickness of0.3 μm was formed on the exposed face other than the p-electrode.

Next, a multilayer film of Rh—Ir—Pt was formed on the p-electrode.

(Second Bonding Layer 11)

On the other hand, a metal substrate with a thickness of 200 μm and madeof a composite consisting of Cu 30% and W 70% was used as a conductivesupporting substrate 10 and a closely sticking layer of Ti, a barrierlayer of Pt and a second eutectic forming layer of Pd with thickness of2,000 Å–3,000 Å–12,000 Å were formed in this order on the surface of themetal substrate 10.

Next, while the first bonding layer 9 and the second bonding layer 11being set face to face, the bonding laminate and the conductivesubstrate were bound by heating and pressing at 250° C. heatertemperature. Accordingly, the metals of the first eutectic forming layerand the second eutectic forming layer were mutually diffused to form aneutectic.

(Removal of Substrate for Growing Nitride Semiconductor 1)

With respect to the laminate for bonding to which the conductivesupporting substrate 10 was bound, laser beam in linear state of 1 mm×50mm was radiated with an output power of 600 J/cm² to the entire surfaceof the opposed face of the sapphire substrate 1 from the under layerside by scanning using KrF excimer laser with wavelength of 248 nm. Thenitride semiconductor of the under layer 2 was decomposed by the laserradiation to remove the sapphire substrate 1. Further polishing wascarried out until the n-type Al_(0.3)Ga_(0.7)N clad layer was made asthin as about 2.2 μm thickness to eliminate surface roughness.

(n-Electrode)

Next, a multilayer electrode of Ti—Al—Ni—Au was formed on the n-typeclad layer 5 functioning also as the n-type contact to form ann-electrode 12. After that, the conductive supporting substrate 10 waspolished to a thickness of 100 μm and then as a pad electrode 13 for thep-electrode, a multilayer film of Ti—Pt—Au—Sn—Au in a thickness of 2,000Å–3,000 Å–3,000 Å–30,000 Å–1,000 Å was formed on the rear face of theconductive supporting substrate 10. Finally, devices were separated bydicing. As illustrated in FIGS. 5A and 5B, the n-electrode and thep-electrode were formed in lattice-like forms in the entire surface ofthe respective semiconductor layer faces. In this case, the apertureparts in the lattice forms are formed reciprocally so as not to beoverlapped with each other in the n-side and the p-side.

The obtained LED was in size of 1 mm×1 mm, emitted UV luminescence with365 nm wavelength at 20 mA in forward direction and had output power of2.4 mW and Vf of 3.6 V.

Example 7

In this Example, the invention was applied to a blue emitting LEDdifferent from UV emitting LED of the Examples 1 to 6. In the Example,since the luminescence wavelength was as long as 460 nm, self-absorptionof the luminescence by a GaN layer was scarcely caused. Accordingly,differing from the Examples 1 to 6, the GaN layer, which was a hightemperature grown layer, could be left to utilize it as an n-typecontact layer.

(Substrate for Growing Nitride Semiconductor)

A substrate of sapphire (C-plane) was used as a substrate for growingnitride semiconductor and surface cleaning was carried out at 1,050° C.in hydrogen atmosphere in a MOCVD reaction vessel.

(Under Layer)

Buffer layer: Successively, a buffer layer 2 of GaN in a thickness ofabout 200 Å was grown on the substrate at 510° C. in hydrogen atmosphereusing ammonia and TMG (trimethylgallium).

(n-Type Contact Layer)

After growth of the buffer layer, an n-type contact layer of n-type GaNin 5 μm thickness and doped with Si in a concentration of 1×10¹⁸/cm³ wasgrown by stopping only TMG supply, increasing the temperature to 1,050°C., and using TMG, ammonia, and silane as raw material gases when itreached at 1,050° C.(n-Type Clad Layer)

Next, an n-type clad layer 5 of n-type Al_(0.18)Ga_(0.82)N doped with Siin a concentration of 5×10¹⁷/cm³ and with a thickness of 400 Å was grownby using TMG, TMA, ammonia, and silane as raw material gases at 1,050°C.

(Active Layer)

Next, barrier layers of GaN doped with Si and well layers of undopedInGaN were laminated in the order of barrier layer/well layer/barrierlayer/well layer/barrier layer by controlling the temperature at 800° C.and using TMI, TMG, and TMA as raw material gases. In this case, thethickness was controlled to be 200 Å for the barrier layers and 50 Å forthe well layers. The active layer had the total thickness about 700 Åand a multilayer quantum well (MQW) structure.

(p-Type Clad Layer)

Next, a p-type clad layer 7 of Al_(0.2)Ga_(0.8)N doped with Mg in aconcentration of 1×10²⁰/cm³ and with a thickness of 600 Å was grown byusing TMG, TMA, ammonia, and Cp₂Mg (cyclopentadienylmagnesium) at 1,050°C. in hydrogen atmosphere.

(p-Type Contact Layer)

Successively, a p-type contact layer of GaN doped with Mg in aconcentration of 2×10²¹/cm³ and with a thickness of 0.15 μm was grown onthe p-type clad layer by using TMG, ammonia, and Cp₂Mg.

On completion of the growth, the wafer was annealed at 700° C. in thereaction vessel in nitrogen atmosphere to further lower the resistanceof the p-type layer.

(First Bonding Layer)

After annealing, the wafer was taken out of the reaction vessel and a Rhfilm in a thickness of 2,000 Å was grown on the p-type contact layer toform a p-electrode. After that, ohmic annealing was carried out at 600°C. and then an insulating protection film SiO₂ in a thickness of 0.3 μmwas formed on the exposed face other than the p-electrode.

Next, a multilayer film of a multilayer film of Ni—Pt—Au—Sn—Au in athickness of 2,000 Å–3,000 Å–3,000 Å–30,000 Å–1,000 Å was formed on thep-electrode. Here, Ni was a bonding layer, Pt was a barrier layer, Snwas a first eutectic forming layer, the Au layer between the Pt and Snwas a layer for preventing diffusion of Sn to the barrier layer, and theoutermost Au layer was a layer for improving adhesion strength to asecond eutectic forming layer.

(Second Bonding Layer)

On the other hand, a metal substrate with a thickness of 200 μm and madeof a composite consisting of Cu 30% and W 70% was used as a conductivesubstrate and a closely sticking layer of Ti, a barrier layer of Pt anda second eutectic forming layer of Au with thickness of 2,000 Å–3,000Å–12,000 Å were formed in this order on the surface of the metalsubstrate.

Next, while the first bonding layer and the second bonding layer beingset face to face, the bonding laminate and the conductive substrate werebound by heating and pressing at 250° C. heater temperature.Accordingly, the metals of the first eutectic forming layer and thesecond eutectic forming layer were mutually diffused to form aneutectic.

Next, with respect to the laminate form bonding to which the conductivesubstrate was bound, laser beam in linear state of 1 mm×50 mm wasradiated with an output power of 600 J/cm² to the entire surface of theopposed face of the sapphire substrate from the under layer side byscanning using KrF excimer laser with wavelength of 248 nm. The nitridesemiconductor of the under layer was decomposed by the laser radiationto remove the sapphire substrate. Further polishing was carried outuntil the n-type contact layer was exposed to eliminate surfaceroughness.

(n-Electrode)

Next, a multilayer electrode of Ti—Al—Ti—Pt—Au in a thickness of 100Å–2,500 Å–1,000 Å–2,000 Å–6,000 Å was formed on the n-type contact layerto form an n-electrode. After that, the conductive substrate waspolished to a thickness of 100 μm and then as a pad electrode for thep-electrode, a multilayer film of Ti—Pt—Au in a thickness of 1,000Å–2,000 Å–3,000 Å was formed on the rear face of the conductivesubstrate. Finally, devices were separated by dicing.

The obtained LED was in size of 1 mm×1 mm, emitted blue color luminancewith 460 nm wavelength at 20 mA in forward direction.

Example 8

A nitride semiconductor device was fabricated under the same conditionsas those of Example 7. Further, a coating layer of SiO₂ containing YAGas a phosphor was formed on the entire surface of the nitridesemiconductor device.

Accordingly, a nitride semiconductor light emitting device capable ofemitting white luminescence was obtained.

Example 9

A nitride semiconductor device was fabricated under the same conditionsas those of Example 7 and in this Example, a plurality of nitridesemiconductor devices were formed in dot-like arrangement on aconductive substrate. A plurality of the nitride semiconductor deviceswere packaged while exposed face being formed in some portions. Further,a coating layer of SiO₂ containing YAG as a phosphor was formed on theexposed face.

Accordingly, a nitride semiconductor light emitting apparatus comprisinga plurality of arranged nitride semiconductor devices capable ofemitting white luminescence, having a large surface area, and capable ofemitting white luminescence was obtained. The apparatus was usable as alight source for luminescence.

Example 10

In this Example, the invention was applied to a light emitting diodewith 365 nm wavelength to fabricate a nitride semiconductor devicecomprising a layer constitution comprising an n-type composition-gradedlayer as illustrated in FIGS. 8 and 9 and having an electrode structureas illustrated in FIG. 5.

(Substrate for Growing Nitride Semiconductor)

A substrate of sapphire (C-plane) was used as a substrate for growingnitride semiconductor and surface cleaning was carried out at 1,050° C.in hydrogen atmosphere in a MOCVD reaction vessel.

(Under Layer 2)

Buffer layer: Successively, a buffer layer 2 of GaN in a thickness ofabout 200 Å was grown on the substrate at 510° C. in hydrogen atmosphereusing ammonia and TMG (trimethylgallium).

High temperature grown layer: After growth of the buffer layer, a hightemperature grown nitride semiconductor of undoped GaN in 5 μm thicknesswas grown by stopping only TMG supply, increasing the temperature to1,050° C., and using TMG and ammonia as raw material gases when itreached at 1,050° C.

(Composition-Graded Layer 26)

Composition-graded layer: After the high temperature grown layer growth,a composition-graded AlGaN layer 26 in a thickness of 0.4 μm was grownat the same temperature using TMG, TMA, and ammonia as raw materialgases. The composition-graded AlGaN layer 26 was for moderating thelattice unconformity between the high temperature grown layer and then-type clad layer and formed in the manner that the mixed crystal ratioof Al and the doping amount of Si were gradually increased from theundoped GaN to the n-type Al_(0.07)Ga_(0.93)N doped with Si in aconcentration of 1×10¹⁹/cm³

(n-Type Clad Layer 5)

Next, an n-type clad layer 5 of n-type Al_(0.07)Ga_(0.93)N doped with Siin a concentration of 1×10¹⁹/cm³ and with a thickness of 2.5 μm wasgrown by using TMG, TMA, ammonia, and silane at 1,050° C.

(Active Layer 6)

Next, barrier layers of Al_(0.09)Ga_(0.91)N doped with Si in aconcentration of 1×10¹⁹/cm³ and well layers of undopedIn_(0.01)Ga_(0.99)N were laminated in the order of barrier layer(1)/well layer (1)/barrier layer (2)/well layer (2)/barrier layer(3)/well layer (3)/barrier layer (4) by controlling the temperature at900° C. and using TMI (trimethylindium), TMG, and TMA as raw materialgases. In this case, the thickness was controlled to be 200 Å for thebarrier layers (1), (2), (3) and (4) and 60 Å for the well layers (1),(2), and (3). Only the barrier layer (4) was undoped.

(p-Type Clad Layer 7)

Next, a p-type clad layer 7 of Al_(0.38)Ga_(0.62)N doped with Mg in aconcentration of 1×10²⁰/cm³ and with a thickness of 270 Å was grown byusing TMG, TMA, ammonia, and Cp₂Mg (cyclopentadienylmagnesium) at 1,050°C. in hydrogen atmosphere.

(p-Type Contact Layer 8)

Successively, a first p-type contact layer of Al_(0.07)Ga_(0.93)N dopedwith Mg in a concentration of 4×10¹⁸/cm³ and with a thickness of 0.1 μmwas grown on the p-type clad layer 7 by using TMG, TMA, ammonia, andCp₂Mg and after that, a second p-type contact layer ofAl_(0.07)Ga_(0.93)N doped with Mg in a concentration of 1×10²⁰/cm³ andwith a thickness of 0.02 μm was grown by adjusting the gas flow rates.

On completion of the growth, in nitrogen atmosphere, the wafer wasannealed at 700° C. in the reaction vessel to further lower theresistance of the p-type layers 7 and 8.

(First Bonding Layer 9)

After annealing, the wafer was taken out of the reaction vessel and a Rhfilm in a thickness of 2,000 Å was formed on the p-type contact layer toform a p-electrode. After that, ohmic annealing was carried out at 600°C. and then an insulating protection film SiO₂ in a thickness of 0.3 μmwas formed on the exposed face other than the p-electrode.

Next, a multilayer film of Rh—Ir—Pt was formed on the p-electrode.

(Second Bonding Layer 11)

On the other hand, a metal substrate with a thickness of 200 μm and madeof a composite consisting of Cu 15% and W 85% was used as a conductivesubstrate 10 and a closely sticking layer of Ti, a barrier layer of Ptand a second eutectic forming layer of Pd with thickness of 2,000Å–3,000 Å–12,000 ↑1 were formed in this order on the surface of themetal substrate 10.

Next, while the first bonding layer 9 and the second bonding layer 11being set face to face, the bonding laminate and the conductivesubstrate 10 were bound by heating and pressing at 230° C. heatertemperature. Accordingly, the metals of the first eutectic forming layerand the second eutectic forming layer were mutually diffused to form aneutectic.

(Removal of Substrate for Growing Nitride Semiconductor)

With respect to the laminate for bonding to which the conductivesubstrate 10 was bound, laser beam in linear state of 1 mm×50 mm wasradiated with an output power of 600 J/cm² to the entire surface of theopposed face of the sapphire substrate 1 from the under layer side byscanning using KrF excimer laser with wavelength of 248 nm. The nitridesemiconductor of the under layer 2 was decomposed by the laser radiationto remove the sapphire substrate 1. Further polishing was carried outuntil the n-type clad layer 5 composed of the under layer 2, thecomposition-graded layer 26 and the n-type Al_(0.3)Ga_(0.7)N was made asthin as about 2.2 μm thickness to eliminate surface roughness.

(n-Electrode 2)

Next, a multilayer electrode of Ti—Al—Ni—Au was formed on the n-typecontact layer to form an n-electrode 12. In consideration of the lightoutputting efficiency, the n-electrode 12 was formed not on the entiresurface but in portions so as to have 70% aperture ratio. After that,the conductive substrate 10 was polished to a thickness of 100 μm andthen as a pad electrode 13 for the p-electrode, a multilayer film ofTi—Pt—Au—Sn—Au in a thickness of 2,000 Å–3,000 Å–3,000 Å–30,000 Å–1,000Å was formed on the rear face of the conductive substrate 10. Finally,devices were separated by dicing. As illustrated in FIGS. 5A and 5B, then-electrode 12 and the p-electrode 16 were formed in lattice-like formsin the entire surface of the respective semiconductor layer faces. Inthis case, the aperture parts in the lattice forms are formedreciprocally so as not to be overlapped with each other in the n-sideand the p-side.

The characteristics of the obtained LED were as shown in FIG. 11.

FIG. 11 shows alteration of the operation voltage (Vf) and the output inrelation to the forward current in cases of pulsed current (Pulse) anddirect current (DC). The device emitted UV luminescence with 365 nmwavelength and had output power of 118 mW, operation voltage of 4.9 V,and external quantum efficiency of 6.9% with 500 mA pulsed electriccurrent (pulse width of 2 μsec, duty 1%) at a room temperature andemitted UV luminescence with 365 nm wavelength and had output power of100 mW, operation voltage of 4.6 V, and external quantum efficiency of5.9% with 500 mA direct current at a room temperature.

Comparative Example 1

(Substrate for Growing Nitride Semiconductor)

A substrate of sapphire (C-plane) was used as a substrate for growingnitride semiconductor and surface cleaning was carried out at 1,050° C.in hydrogen atmosphere in a MOCVD reaction vessel.

(Under Layer)

Buffer layer: Successively, a buffer layer 2 of GaN in a thickness ofabout 200 Å was grown on the substrate at 510° C. in hydrogen atmosphereusing ammonia and TMG (trimethylgallium).

High temperature grown layer: After growth of the buffer layer, a hightemperature grown nitride semiconductor of undoped GaN in 5 μm thicknesswas grown by stopping only TMG supply, increasing the temperature to1,050° C., and using TMG and ammonia as raw material gases when itreached at 1,050° C.

Composition-graded layer: After the high temperature grown layer growth,a composition-graded AlGaN layer in a thickness of 0.4 μm was grown atthe same temperature using TMG, TMA, and ammonia as raw material gases.The composition-graded AlGaN layer was for moderating the latticeunconformity between the high temperature grown layer and the n-typeclad layer and formed in the manner that the mixed crystal ratio of Aland the doping amount of Si were gradually increased from the undopedGaN to the n-type Al_(0.07)Ga_(0.93)N doped with Si in a concentrationof 1×10¹⁹/cm³

(n-type Clad Layer)

Next, an n-type clad layer 5 of n-type Al_(0.07)Ga_(0.93)N doped with Siin a concentration of 1×10¹⁹/cm³ and with a thickness of 2.5 μm wasgrown by using TMG, TMA, ammonia, and silane at 1,050° C.

(Active Layer)

Next, barrier layers of Al_(0.07)Ga_(0.93)N doped with Si in aconcentration of 1×10¹⁹/cm³ and well layers of undopedIn_(0.07)Ga_(0.93)N were laminated in the order of barrier layer(1)/well layer (1)/barrier layer (2)/well layer (2)/barrier layer(3)/well layer (3)/barrier layer (4) by controlling the temperature at900° C. and using TMI (trimethylindium), TMG, and TMA as raw materialgases. In this case, the thickness was controlled to be 200 Å for thebarrier layers (1), (2), (3) and (4) and 60 Å for the well layers (1),(2), and (3). Only the barrier layer (4) was undoped.

(p-Type Clad Layer)

Next, a p-type clad layer 7 of Al_(0.38)Ga_(0.62)N doped with Mg in aconcentration of 1×10²⁰/cm³ and with a thickness of 270 Å was grown byusing TMG, TMA, ammonia, and Cp₂Mg (cyclopentadienylmagnesium) at 1,050°C. in hydrogen atmosphere.

(p-Type Contact Layer)

Successively, a second p-type contact layer of Al_(0.07)Ga_(0.93)N dopedwith Mg in a concentration of 4×10¹⁸/cm³ and with a thickness of 0.1 μmwas grown on the p-type clad layer by using TMG, TMA, ammonia, and Cp₂Mgand after that, a second p-type contact layer of Al_(0.07)Ga_(0.93)Ndoped with Mg in a concentration of 1×10²⁰/cm³ and with a thickness of0.02 μm was grown by adjusting the gas flow rates.

On completion of the growth, in nitrogen atmosphere, the wafer wasannealed at 700° C. in the reaction vessel to further lower theresistance of the p-type layers.

(Electrode)

A portion of the second p-type contact layer was etched until the n-typeclad layer was exposed to form a face to form an n-electrode thereon anda Rh electrode was formed on the second p-type contact layer andTi—Al—Ni—Au was formed on the exposed n-type clad layer.

The obtained LED showed the luminescence spectrum shown in FIG. 12A.FIG. 12B shows the luminescence spectrum of the device obtained inExample 10, and it was found that absorption of GaN greatly affected theluminescence spectra.

Example 11

An LED was fabricated in the same manner as Example 10 except that then-clad layer 5 is divided into two layers as shown in FIG. 10.

(n-Type Clad Layer 5)

A first n-type nitride semiconductor layer 5 a of n-typeAl_(0.07)Ga_(0.93)N doped with Si in a concentration of 1×10¹⁹/cm³ andwith a thickness of 1.7 μm was grown using TMG, TMA, ammonia, and silaneat 1,050° C. and further thereon, a second n-type nitride semiconductorlayer 5 b of n-type Al_(0.07)Ga_(0.93)N doped with Si in a concentrationof 2×10¹⁷/cm³ and with a thickness of 0.8 μm was grown to form an n-typeclad layer 5 composed of the first n-type nitride semiconductor layer 5a and the second n-type nitride semiconductor layer 5 b.

The LED obtained accordingly had operation voltage lowered further byabout 0.3 V than the LED of Example 10 and the deterioration was slighteven after long time light emission.

Example 12

This Example was carried out under same conditions as those of Example11 except that the active layer is formed in the following condition.

(Active Layer 6)

Next, after setting the temperature at about 900° C., using TMI(trimethylindium), TMG, and TMA as raw material gases, barrier layers ofAl_(0.09)Ga_(0.91)N doped with Si in a concentration of 1×10¹⁹/cm³ andwell layers of undoped In_(0.01)Ga_(0.99)N were laminated in the orderof barrier layer (1)/well layer (1)/barrier layer (2)/well layer(2)/barrier layer (3)/well layer (3)/barrier layer (4)/well layer(4)/barrier layer (5)/well layer (5)/barrier layer (6). In this case,the thickness was controlled to be 200 Å for the barrier layers (1) to(6) and 150 Å for the well layers (1) to (5).

Thus provided LED device has an output power substantially equal to thatin Example 11 and light emitting spectrum improved in terms ofmonochromatism.

Example 13

In Example 10, projected and recessed parts in stripes were formed inthe exposed n-type contact layer after the formation of the n-typeelectrode. The depth and the width of the recessed parts were adjustedto 1.5 μm and 3 μm, respectively and the width of the projected partswas adjusted to be 3 μm. Other conditions were same as those in Example10. This dimpling process further improved the luminescence outputtingefficiency.

Example 14

With respect to the device obtained by Example 10 or Example 11, afterformation of the n-type electrode, a protection film 19 of an insulatingZrO₂ (refractive index 2.2) with a thickness of 1.5 μm was formed.Further, as shown in FIG. 13, projected and recessed parts were formedin the protection film 19 at 3 μm intervals. The plane shape of eachprojected part was circular and the depth of the recessed parts wasadjusted to be 1.0 μm. Accordingly, the luminescence outputtingefficiency was further improved. Although ZrO₂ was used in this Example,similar effects could be obtained in the case of using Nb₂O₅ (refractiveindex 2.4) and TiO₂ (refractive index 2.7).

Example 15

In Example 14, the projected parts to be formed in the protection film19 were made to be projected parts with a shape having a tapered angleof 60° C. in the ZrO₂ protection film. According to this Example, theluminescence outputting efficiency was further improved as compared withthat of Example 13.

Example 16

An LED was obtained in the similar method as Example 10. Successively,the LED was mounted on a metal package and electric communication of theLED with an external electrode was preformed by a conductive wire andafter that, a coating layer 14 shown in FIG. 14 was formed on thenitride semiconductor-containing light emitting device by the followingmethod.

(1) At first, a resist or a polyimide film was formed on the electrodeof the LED. (2) Next, as described above, a coating solution wasproduced by producing a mixed solution of hydrolytic solutions ofcerium-activated yttrium aluminum garnet type phosphor and ethylsilicate with a high boiling point solvent and stirring the resultingsolution mixture as to evenly disperse the phosphor. (3) The coatingsolution was applied to the top face and the side faces of the LEDexcept the supporting substrate and the portion where the protectionfilm was formed by a spray coating method. (4) The coating was primarilycured by drying at 150° C. for 30 minutes to form a layer with athickness of several ten μm. (5) The formed layer was impregnated withthe hydrolytic solution of ethyl silicate containing no phosphor. (6)Finally, the resist or the polyimide film was removed and secondarycuring was carried out by drying at 240° C. for 30 minutes. Through theabove-mentioned steps (1) to (6), a coating layer 14 with anapproximately even thickness of 20 to 30 μm, which was a continuouslayer existing at least on the exposed face of the nitride semiconductorlayers with the total thickness of 5 to 10 μm and positioned in theupper face and side faces of the LED except the electrode, was formed.

The light emitting apparatus obtained according to the Example wasprovided with the phosphor, an inorganic material scarcely deterioratedeven if used together with a light emitting device emitting luminescencewith wavelength in a range from blue to UV rays, firmly stuck to thelight emitting device and thus a light emitting apparatus with littleunevenness of luminescence color even in the case of long time use couldbe obtained. Further, the light emitting apparatus according to theExample could be a light emitting apparatus with little change of colortemperature even from different luminescence observation directionsowing to the formation of the coating layer 14 which had anapproximately even thickness and covered at least the luminescenceobservation faces of the light emitting device. Further, since thecoating layer containing the phosphor was formed on the all of the faceswhere the luminescence from the light emitting device, no light wastransmitted through the supporting substrate, and therefore, as comparedwith a conventional light emitting device using a sapphire substrate,the outputting efficiency of the luminescence with wavelength changed bythe phosphor can be improved. Further, use of a supporting substratewith a high thermal conductivity improved heat releasing property ascompared with that of a conventional light emitting device using asapphire substrate.

Example 17

After the side faces of respective p- and n-semiconductors on thesupporting substrate were exposed by etching as shown in FIG. 14, acoating layer was formed by screen printing using a coating solutionproduced in the same manner as Example 16 or a material obtained byadding a cerium-activated yttrium aluminum garnet type phosphor to asilicone resin. Here, in the case of using the material obtained byadding the phosphor to a silicone resin, the curing was carried out at150° C. for 1 hour. The semiconductor wafer obtained in such a mannerwas divided by dicing after a scribing line was formed in the wafer toobtain LED chips as light emitting devices.

In such a manner, formation of the coating layer 14 containing aphosphor in the wafer state made it possible to carry out inspection andselection of luminescence color before fabrication of a light emittingapparatus by disposing the LED chips in a metal package or the like,that is, in the stage of the LED chips bearing the coating layercontaining the phosphor, so that the production yield of the lightemitting apparatus could be improved. Further, the LED chips accordingto the Example could be light emitting devices with little change ofcolor temperature even from different luminescence observationdirections to the LED chips bearing the above-mentioned coating layer14.

Example 18

FIG. 15 shows a schematical cross-sectional view of a semiconductorlight emitting device according to this Example.

After the removal of a substrate for growing nitride semiconductor inExample 16, in order to improve the luminescence outputting efficiency,projected and recessed parts (dimples) were formed in the exposed faceof the n-type nitride semiconductor layers and/or side faces of thesemiconductor layers by RIE. In this Example, as shown in FIG. 15,dimpling process was carried out especially for the n-type clad layer 5,however the dimpling process could be carried out from the n-type cladlayer 5 to the active layer 6 and from the p-type clad layer 7 to thep-type contact layer 8. The cross-sectional shape of the projected andrecessed parts could be mesa type or reverse mesa type and the planeshape could be island-like, lattice-like, rectangular, circular, orpolygonal shape. The coating layer 14 similar to that of Example 15 wasformed on the exposed face and the side faces of the semiconductorlayers subjected to dimpling process.

A light emitting apparatus with improved luminescence outputtingefficiency from light emitting devices and little unevenness ofluminescence color even in the case of long time use could be fabricatedby formation of the coating layer in such a manner.

Example 19

A nitride semiconductor device was fabricated in this Example under thesame conditions as those of Example 16, however a plurality of nitridesemiconductor devices were formed on a supporting substrate while beingarranged like dots. Exposed faces were formed in portions of the sidefaces of the nitride semiconductor devices by etching and the coatinglayer 14 was formed in the same manner as that of Example 15. Finally,the supporting substrate was disposed on a support such as a metalpackage and pairs of both negative and positive electrodes of the lightemitting devices were electrically communicated with external electrodesto fabricate a light emitting apparatus.

Accordingly, a plurality of light emitting devices capable of emittingmixed color light obtained by mixing the luminescence from the lightemitting devices and luminescence obtained by changing the wavelength ofthe luminescence from the light emitting devices by the phosphor werearranged to give a light emitting apparatus with a large surface areaand capable of emitting the mixed color light. Such a light emittingapparatus can be used as a light source for luminaire.

Example 20

In this Example, the semiconductor light emitting device obtained inExample 16 was disposed in a package shown in FIG. 7. The semiconductorlight emitting device 30 was die-bonded by an epoxy resin in the bottomface of an aperture part of a package 40 provided with a heat sink. Thebonding material to be used for the die bonding was not particularlylimited and a resin or glass containing Au—Sn alloy or a conductivematerial could be used. The conductive material to be added ispreferably Ag and if Ag paste with 80 to 90% content was used, a lightemitting apparatus excellent in heat releasing property and havingsuppressed stress after the bonding can be obtained. Next, therespective electrodes of the die-bonded semiconductor light emittingdevice 30 and the respective lead electrodes 44 exposed out of thebottom face of the aperture of the package 40 were electricallyconnected to each other through respective Au wires 46.

Next, 3% by weight of lightweight calcium carbonate (refractive index1.62) with an average particle size of 1.0 μm and an oil absorptioncapability of 70 ml/100 g as a dispersant was added to 100% by weight ofa phenyl methyl type silicone resin composition (refractive index of1.53) and stirred for 5 minutes by a planetary mixer. Next, to releaseheat generate by the stirring treatment, the resin was left still for 30minutes to cool to a constant temperature and stabilized. The curablecomposition 48 obtained in such a manner was packed in the aperture partof the package 40 to the line of the same plane of the top faces of bothends of the aperture. Finally, the composition was subjected to heatingtreatment of 70° C.×3 hours and 150° C.×1 hour. Accordingly, a lightemitting face with a right and left symmetric and parabolic concavedshape from the top faces of both ends of the aperture toward the centerpart can be obtained. The potting member 48 made of the cured materialof the curable composition was separated into two layers; a first layerwith a high dispersant content and a second layer with a lowerdispersant content than that of the first layer and the surface of thesemiconductor light emitting device was coated with the first layer.Accordingly, luminescence emitted out of the semiconductor lightemitting device could be taken to the outside and the evenness of theluminescence could be improved. The first layer was preferably formedcontinuously from the bottom face of the aperture part to the surface ofthe semiconductor light emitting device and consequently, the shape ofthe light emitting face could be the smooth aperture part. The lightemitting device according to the Example could emit luminescence fromthe light emitting device from the main face side without vain and ascompared with a conventional device, it could be made thin and radiatelight rays to a wide range of a light impingent face of a photo-guidingboard.

Example 21

A light emitting device was fabricated in the same manner as Example 20,except that the phosphor substance was added in the potting material 48.The phosphor substance could be obtained in the following manner.Coprecipitation was caused in a solution, which was obtained bydissolving rare earth metals such as Y, Gd, and Ce in stoichiometricratios in an acid, by adding oxalic acid and the oxides of thecoprecipitates obtained by firing the coprecipitates were mixed withaluminum oxide to obtain a raw material mixture. After being mixed withbarium fluoride as a flux, the obtained raw material mixture was packedin a crucible and fired at 1,400° C. for 3 hours in air to obtain afired product and the fired product was ball-milled in water, washed,separated, dried, and finally sieved to obtain the phosphor substance of(Y_(0.995)Gd_(0.005))_(2.750)Al₅₀₁₂:Ce_(0.250) with a mean particle sizeof 8 μm. Addition of the phosphor could provide a light emitting devicecapable of emitting mixed color light of a portion of the luminescenceof the light emitting apparatus with the luminescence obtained byconversion of wavelength of the luminescence by the phosphor.

Example 22

In this Example, a light emitting apparatus is formed by packaging thedevice of Example 16 with phosphor materials according to the packagingmethod described in this specification. BaAl₁₂O₁₉:Mn and Sr₄Al₁₄O₂₅:Euare used as phosphor materials. This apparatus emits blue-green light.

Example 23

This Example will describe a laser fabricated by applying the inventionwith reference to FIGS. 16A to 16L. First, as shown in FIG. 16A, n-typenitride semiconductor layers 62, an active layer 63 and p-typesemiconductor layers are formed on a sapphire substrate 61.

(Buffer Layer)

In this example, a sapphire was used as a substrate for growing nitridesemiconductor 61. At first, a hetero substrate 61 made of a sapphirehaving the c-plane as a main face, and 2 inch Φ was set in a MOVPEreaction vessel and a buffer layer of GaN with a thickness of 200 Å wasgrown using trimethylgallium (TMG), ammonia (NH₃) at 500° C.

(Under Layer)

After buffer layer formation, a nitride semiconductor layer of undopedGaN with a thickness of 4 μm was grown using TMG and ammonia at 1,050°C. The layer worked as an under layer (a substrate for growing nitridesemiconductor) for growing respective layers composing the devicestructure. A substrate for growing nitride semiconductor with goodcrystallinity could be obtained if a nitride semiconductor grown by ELOG(Epitaxially Laterally Overgrowth) besides the above-mentioned layer. Apractical example of the ELOG layer includes layers formed by forming anitride semiconductor layer on a hetero substrate, forming stripes ofmask regions on the surface by forming protection films on which anitride semiconductor is difficult to grow and non-mask regions on whichthe nitride semiconductor is to be formed, and growing the nitridesemiconductor from the non-mask regions not only in the thicknessdirection but also in the lateral direction so as to grow the nitridesemiconductor even on the mask regions and also by forming apertures inthe nitride semiconductor layer grown on the hetero substrate andpromoting growth of the layer in the lateral direction from the sidefaces of the apertures.

Next, the respective layers composing the laminate structure were formedon the under layer of the nitride semiconductor.

(n-Type Contact Layer)

Successively, an n-type contact layer of GaN doped with Si in aconcentration of 4.5×10¹⁸/cm³ and with a thickness of 2.25 μm was grownat 1,050° C. by similarly using TMG, ammonia, and silane as an impuritygas. The thickness of the n-contact layer was satisfactory to be 2 to 30μm.

(Crack Prevention Layer)

Next, a crack prevention layer of In_(0.06)Ga_(0.94)N with a thicknessof 0.15 μm was grown at 800° C. by using TMG, TMI (trimethylindium) andammonia. The crack prevention layer may be omitted.

(n-Type Clad Layer)

Next, an A layer of undoped AlGaN with a thickness of 25 Å was grown at1,050° C. by using TMA (trimethyaluminum), TMG, and ammonia as rawmaterial gases and successively the supply of TMA was stopped, and a Blayer of GaN doped with Si in a concentration of 5×10¹⁸/cm³ and with athickness of 25 Å was grown using silane gas as an impurity gas. Thesesteps were respectively repeated 160 times to reciprocally form the Alayer and the B layer to grow an n-type clad layer composed ofmultilayered films (superlattice structure) in a total thickness of8,000 Å. In this case if the mixed crystal ratio of Al in the undopedAlGaN was in a range not lower than 0.05 and not higher than 0.3,sufficiently high refractive index difference to work as a clad layercould be obtained.

(n-Type Photo-Guiding Layer)

Next, an n-type photo-guiding layer of undoped GaN with a thickness of0.1 μm was grown at the same temperature using TMG and ammonia as rawmaterial gases. The layer could be doped with an n-type impurity.

(Active Layer)

Next, a barrier layer of In_(0.05)Ga_(0.95)N doped with Si in aconcentration of 5×10¹⁸/cm³ and with a thickness of 100 Å was formed at800° C. using TMI (trimethylindium), TMG, and ammonia as raw materialsand silane gas as an impurity gas. Successively, the silane gas wasstopped and a well layer of undoped In_(0.1)Ga_(0.9)N with a thicknessof 50 Å was formed. These steps were respectively repeated 3 times andfinally the barrier layer was grown to form an active layer having thetotal thickness about 550 Å and a multilayer quantum well structure(MQW).

(p-Type Cap Layer)

Next, at the same temperature, a p-type electron-enclosing layer ofAlGaN doped with Mg in a concentration of 1×10¹⁹/cm³ and with athickness of 100 Å was grown using TMA, TMG, and ammonia as raw materialgases and Cp₂Mg (cyclopentadienylmagnesium) as an impurity gas.

(p-Type Photo-Guiding Layer)

Next, a p-type photo-guiding layer of undoped GaN with a thickness of750 Å was grown at 1,050° C. using TMG and ammonia as raw materialgases. Although the p-type photo-guiding layer was grown while beingundoped, the layer could be doped with Mg.

(p-Type Clad Layer)

Next, a layer of undoped Al_(0.16)Ga_(0.84)N with a thickness of 25 Åwas grown at 1,050° C. and successively the supply of TMA was stopped,and a layer of Mg-doped GaN with a thickness of 25 Å was grown usingCp₂Mg and consequently a p-type clad layer with a superlattice structurein a total thickness of 0.6 μm was grown. In the case the p-type cladlayer was formed in a superlattice structure composed of layered nitridesemiconductor layers of which one type layers were Al-containing nitridesemiconductor layers and have different band gap energy from that of theother, the crystallinity tended to be made excellent if highconcentration doping in one type layers, so-called modulated doping, wasperformed, however doping could be performed similarly in both typelayers.

(p-Type Contact Layer)

Finally, a p-type contact layer of a p-type GaN doped with Mg inconcentration of 1×10²⁰/cm³ and with a thickness of 150 Å was grown onthe p-type clad layer at 1,050° C. The p-type contact layer could be ap-type In_(x)Al_(y)Ga_(1-x-y)N, (x≦, 0 y≦0, x+y≦1) and preferably, ifthe layer was of Mg-doped GaN, the most desirable ohmic contact with thep-electrode could be obtained. On completion of the reaction, the waferwas annealed at 700° C. in nitrogen atmosphere in the reaction vessel tofurther lower the resistance of the p-type layer.

In such a manner, as shown in FIG. 16A, a laminate body composed of then-type nitride semiconductor layer 62, the active layer 63, and thep-type nitride semiconductor layer 64 on the sapphire substrate 61 wasobtained.

(n-Type Layer Exposure And Resonance Face Formation)

After the nitride semiconductor was formed in the above-mentionedmanner, the wafer was taken out of the reaction vessel and a protectionfilm of SiO₂ was formed on the surface of the uppermost layer, thep-type contact layer, and then the wafer was subjected to etching usingSiCl₄ by RIE (reactive ion etching). Accordingly, as shown in FIG. 16B,an active layer end face to be a resonance face was exposed to give theetching end face as a resonance end face.

(Substrate Exposure)

Next, after SiO₂ was formed on the entire surface of the wafer, aresists film was formed thereon except the exposed face of the n-typecontact layer and as shown in FIG. 16C, etching was carried out untilthe substrate was exposed. Since the resist film was formed on the sidefaces of the resonance face, after the etching, end faces in two stepsbetween the side faces (including the p-type layer, the active layer,and a portion of the n-type layer) of the resonance face formed beforeand the n-type layer existing between the resonance face and thesubstrate were formed.

(Stripe-Like Projected Part (Ridge) Formation)

Next, as shown in FIG. 16D, a stripe-state waveguide region was formed.After the protection film of Si oxide (mainly SiO₂) with a thickness of0.5 μm was formed on the entire surface of the uppermost layer, which isthe p-type contact layer, by a CVD apparatus, a mask in stripes with 3μm width was put on the protection film and SiO₂ was etched withy CF₄ bya RIE apparatus and after that, the nitride semiconductor layers wereetched by SiCl₄ until the p-type guiding layer was exposed to formprojected parts 64 in stripes projected more than the active layer.

(First Insulation Film)

While the SiO₂ being put as it was, a first insulating film of ZrO₂ wasformed on the surface of the p-type semiconductor layer 64. Portionswhere on first insulation film was formed could be left, so that thefirst insulation film 65 was easily divided thereafter. After the firstinsulation film formation, the wafer was immersed in a buffered solutionto dissolve and remove SiO₂ formed on the top faces of the projectedparts in stripes and together with SiO₂, the ZnO₂ existing on the p-typecontact layer (further on the n-type contact layer) was removed bylift-off method. Accordingly, the top faces of the projected parts instripes were exposed and the side faces of the projected parts werecovered with ZrO₂.

(p-Side Ohmic Electrode)

Next, as shown in FIG. 16E, a p-side ohmic electrode 65 was formed inthe first insulation film on the uppermost faces of the projected partson the p-type contact layer. The p-side ohmic electrode 65 consisted ofNi and Au. After the electrode formation, annealing at 600° C. wascarried out in atmosphere containing oxygen and nitrogen in 1:99 (O:N)ratio to carry out alloying of the p-side ohmic electrode and obtaingood ohmic characteristics.

(Second Insulation Film)

Next, a resist was applied to a portion of the p-side ohmic electrode onthe projected parts in stripes and a second insulation film with amultilayer structure consisting of two pairs (4 layers) of Si oxide(mainly SiO₂) and Ti oxide (mainly TiO₂) with each film thickness ofλ/4n was formed in the etched bottom face and side faces to form amirror. In this case, the p-side ohmic electrode was kept exposed.

(p-Side Pad Electrode)

Next, as shown in FIG. 16E, a p-side pad electrode 66 was formed so asto cover the above-mentioned insulation films. The p-side pad electrode66 was composed of a closely sticking layer, a barrier layer, and aneutectic layer and the respective layers were formed so as to beRhO—Pt—Au in this order with film thickness of 2,000 Å–3,000 Å–6,000 Åfrom the p-type semiconductor layer side.

(Protection Film Before Sticking First Supporting Substrate)

A resist film 67 with a thickness of 3 μm was formed so as to cover theentire surface of the wafer after formation of the p-side pad electrode66.

On the other hand, a first supporting substrate 68 was made ready. Asthe first supporting substrate 68, sapphire with a thickness of 425 μmwas used. As shown in FIG. 16F), the sapphire and the p-typesemiconductor layer side of the nitride semiconductor layers obtained inthe above-mentioned manner were stuck to each other by inserting anepoxy type bonding sheet 69. In this case, the forgoing nitridesemiconductor was stuck through the protection film formed afterformation of the p-side pad electrode. Bonding was carried out byheating at about 150° C. for about 1 hour by a heating press.

(Separation of Substrate for Growing Nitride Semiconductor)

Next, as shown in FIG. 16G, the sapphire substrate 61, which was asubstrate for growing nitride semiconductor, was removed by polishing.Further, polishing was continued until the n-type contact layer wasexposed. The surface roughness was eliminated by chemical polishingusing KOH and colloidal silica (KSiO₃).

(n-Side Electrode and n-side Metallizing Layer)

Next, as shown in FIG. 16H, an n-side metal layer 70 of Ti—Al—Ti—Pt—Snin this order with thickness of 100 Å–2,500 Å–1,000 Å–2,000 Å–6,000 Åwas formed on the foregoing n-type contact layer. The Ti—Al layer was ann-side electrode and the Ti—Pt—Sn thereon was a metallizing layer foreutectic formation.

On the other hand, a second supporting substrate 71 was made ready. Ametallizing layer 72 of Ti—Pt—Au in this order with thickness of 2,000Å–3,000 Å–12,000 Å was formed on the second supporting substrate 71 witha thickness of 200 μm and consisting of Cu 20% and W 80%.

(Sticking of Second Supporting Substrate)

Next, as shown in FIG. 16I, the foregoing n-side metal layer 70 and themetallizing layer 72 of the second supporting substrate were set face toface and stuck to each other. Pressure was applied at 240° C. At thattime, an eutectic was formed.

(Separation of First Supporting Substrate)

As described above, the wafer of nitride semiconductor layers sandwichedbetween the first supporting substrate 68 and the second supportingsubstrate 71 was heated to about 200° C. Accordingly, the adhesionstrength of the epoxy type adhesion sheet 69 was decreased, so that thefirst supporting substrate 68 could be separated as shown in FIG. 16J.After the protection film (resist) 67 formed on the p-side pad electrodewas removed shown in FIG. 16K, dicing was carried out to obtain chips.

In such a manner as described above, a nitride semiconductor laserdevice shown in FIG. 16L was obtained. The laser device had a thresholdcurrent density of 1.5 kA/cm² and a threshold voltage of 3.5 V.

Example 24

In this Example, same processes to the second protection film formationof Example 23 were carried out.

(Filler)

As shown in FIG. 16F, a p-side pad electrode 66 composed of layeringRhO—Pt—Au in this order was formed on the p-side ohmic electrode 65. Thep-side pad electrode 66 was composed of a closely sticking layer, abarrier layer, and an eutectic layer and the respective layers wereformed in thickness of 2,000 Å–3,000 Å–6,000 Å in RhO—Pt—Au order fromthe p-type semiconductor layer side. Next, a polyimide was applied tothe groove parts formed by etching to make the entire body of the waferflat.

On the other hand, a first supporting substrate 68 was made ready. Asthe first supporting substrate 68, sapphire with a thickness of 425 μmwas used and as an adhesive, an eutectic material of AuSn was used. Asshown in FIG. 16G, the eutectic material was stuck to the p-typesemiconductor layer side of the nitride semiconductor layers obtained inthe above-mentioned manner. In this case, the bonding was carried outthrough the protection film formed after formation of the p-side padelectrode. The bonding was carried out by heating at about 240° C. forabout 10 minutes by a heating press.

(Separation of Substrate for Growing Nitride Semiconductor)

Next, as shown in FIG. 16G, the sapphire substrate 61, which was asubstrate for growing nitride semiconductor, was removed by polishing.Further, polishing was continued until the n-type contact layer wasexposed and the surface roughness was eliminated by chemical polishingusing KOH and colloidal silica (K₂SiO₃).

(n-Side Electrode and n-side Metallizing Layer)

A Si oxide film formed as a second protection film was removed usinghydrofluoric acid. Next, as shown in FIG. 16H, an n-side metal layer 70of Ti—Al—Ti—Pt—Sn in this order with thickness of 100 Å–2,500 Å–1,000Å–2,000 Å–6,000 Å was formed on the n-type contact layer.

On the other hand, a second supporting substrate 71 was made ready. Ametallizing layer 72 of Ti—Pt—Pd in this order with thickness of 2,000Å–3,000 Å–12,000 Å was formed on the second supporting substrate 71 witha thickness of 200 μm and consisting of Cu 20% and W 80%.

(Sticking of Second Supporting Substrate)

Next, as shown in FIG. 16I, the foregoing n-side metal layer 70 and themetallizing layer 72 of the second substrate were set face to face andstuck to each other. Pressure was applied at 240° C. At that time, aneutectic was formed.

(Separation of First Supporting Substrate)

As described above, the wafer of nitride semiconductor layers sandwichedbetween the first supporting substrate 68 and the second supportingsubstrate 71 was heated to about 200° C. Accordingly, the adhesionstrength of the epoxy type adhesion sheet 69 was decreased, so that thefirst supporting substrate 68 could be separated as shown in FIG. 16J.After that, as shown in FIG. 16K, the protection film (the polyimide)formed on the p-side pad electrode was removed and then the polyimidepacked-layer was removed by oxygen plasma and dicing was carried out.

The nitride semiconductor laser device obtained in such a manner had athreshold current density of 1.5 kA/cm² and a threshold voltage of 3.5V.

Example 25

In this example, a multistripe type nitride semiconductor laser devicehaving 30 ridges with 3 μm width at 60 μm intervals was fabricated.Other points were same as those of Example 23. FIG. 17A shows themultistripe type nitride semiconductor laser device. To simplify thedrawing, the number of the ridges is 15 in FIG. 17A. In such amultistripe type, the lateral width (the width of the end face in theperpendicular direction to the ridges) becomes wider as the number ofthe ridges is increased more and the laser device becomes a bar-likelaser device. Formation of a plurality of ridges in a given directiongives high output power. The nitride semiconductor laser obtained inthis Example had a threshold current density of 1.5 kA/cm² and athreshold voltage of 3.5 V at a room temperature. In this case, laserbeam is evenly oscillated from the respective stripes and the maximumoutput was 10 W.

Further, the multistripe type nitride semiconductor laser deviceobtained in this Example can be used while being stacked. For example,as shown in FIG. 17B, bar-type laser devices are stacked so as to form aplurality of ridges in the two-dimensional directions and obtain a stacktype semiconductor laser and consequently a semiconductor laser with asuper high output can be obtained. Each of the multistripe type nitridesemiconductor laser devices has the maximum output of 10 W and in thecase of a stack type semiconductor laser obtained by stackingmultistripe type semiconductor laser devices in 10 layers, the outputpower can be as super high as about 100 W. In this case, each laserdevice had a threshold current density 1.5 kA/cm² and a thresholdvoltage of 3.5 V at a room temperature and laser beam is evenlyoscillated from the respective stripes.

1. A nitride semiconductor device comprising: a substrate having twoopposed main faces and having a thermal expansion coefficient higherthan that of a nitride semiconductor; a bonding layer placed on one mainface of said substrate and comprising an eutectic layer; a p-electrodeon said bonding layer; one or more p-type nitride semiconductor layerselectrically connected to the p-electrode; an active layer comprising atleast a well layer of Al_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0≦b≦1, a+b≦1) anda barrier layer of Al_(c)In_(d)Ga_(1-c-d)N, (0≦c≦1, 0≦d≦1, c+d≦1), theactive layer being on said p-type nitride semiconductor layers; and oneor more n-type nitride semiconductor layers on said active layer; an-electrode electrically connected to one or more of said n-type nitridesemiconductor layers; wherein said p-type nitride semiconductor layers,said active layer and said n-type nitride semiconductor layers aresandwiched between said p-electrode and said n-electrode, and whereinsaid n-type nitride semiconductor layers contain Al at least in a partof said n-type nitride semiconductor layers that is in contact with saidn-electrode.
 2. The nitride semiconductor device according to claim 1,wherein said substrate is conductive.
 3. The nitride semiconductordevice according to claim 1, wherein said substrate includes one or moremetals selected from Ag, Cu, Au and Pt and one or more metals selectedfrom W, Mo, Cr and Ni.
 4. The nitride semiconductor device according toclaim 1, wherein said substrate is made of a metal composite comprisingtwo or more metals, wherein the two or more metals have substantially nosolid solubility in each other.
 5. The nitride semiconductor deviceaccording to claim 1, wherein said substrate is a composite of a metaland a ceramic.
 6. The nitride semiconductor device according to claim 1,wherein said nitride semiconductor device comprises an n-electrodecontacting with the surface of said n-type nitride semiconductor layers,and wherein said n-type nitride semiconductor layers comprising at leasttwo layers; a first n-type nitride semiconductor layer doped with ann-type impurity as a layer contacting with said n-electrode and a secondn-type nitride semiconductor layer un-doped or doped with an n-typeimpurity in a less amount than that of said first n-type nitridesemiconductor layer near to said active layer side than said firstn-type nitride semiconductor layer.
 7. The nitride semiconductor deviceaccording to claim 1, wherein said p-type nitride semiconductor layersinclude a p-type contact layer of Al_(f)Ga_(1-f)N, (0<f<1).
 8. Thenitride semiconductor device according to claim 7, wherein said p-typecontact layer has a graded composition with a high p-type impurityconcentration and a small mixed crystal ratio of Al in the substrateside.
 9. The nitride semiconductor device according to claim 8, whereinsaid p-type contact layer is composed of two layers and said two layersare an Al_(g)Ga_(1-g)N, (0<g<0.05) layer formed adjacent to saidp-electrode and an Al_(h)Ga_(1-h)N, (0<h<0.1) layer formed adjacent tothe active layer side and said Al_(g)Ga_(1-g)N, (0<g<0.05) layer has ahigher p-type impurity concentration than that of said Al_(h)Ga_(1-h)N,(0<h<0.1) layer.
 10. The nitride semiconductor device according to claim1, wherein said nitride semiconductor device is a light emitting deviceand comprises a coating layer containing a phosphor substance capable ofabsorbing light from said light emitting device and emittingluminescence with wavelength difference from that of the light emittingdevice and said coating layer is formed in at least a portion of thesurface of said light emitting device.
 11. A nitride semiconductordevice comprising: a substrate having two opposed main faces; a bondinglayer placed on one main face of said substrate and comprising aneutectic layer; a p-electrode on said bonding layer; one or more p-typenitride semiconductor layers electrically connected to the p-electrode;an active layer comprising at least a well layer ofAl_(a)In_(b)Ga_(1-a-b)N, (0≦a≦1, 0≦b≦1, a+b≦1) and a barrier layer ofAl_(c)In_(d)Ga_(1-c-d)N, (0≦c≦1, 0≦d≦1, c+d≦1), the active being on saidp-type nitride semiconductor layers; and n-type nitride semiconductorlayers on said active layer an n-electrode electrically connected to oneor more of said n-type nitride semiconductor layers; wherein said p-typenitride semiconductor layers, said active layer and said n-type nitridesemiconductor layers are sandwiched between said p-electrode and saidn-electrode, and wherein at least a part of said n-type nitridesemiconductor layers, the part that is in contact with said n-electrode,is made of a nitride semiconductor which does not substantially absorbthe light emitted from said active layer.
 12. The nitride semiconductordevice according to claim 11, wherein said nitride semiconductor devicecomprises an n-electrode contacting with the surface of said n-typenitride semiconductor layers and wherein said n-type nitridesemiconductor layers comprising at least two layers; a first n-typenitride semiconductor layer doped with an n-type impurity as a layercontacting with said n-electrode and a second n-type nitridesemiconductor layer un-doped or doped with an n-type impurity in a lessamount than that of said first n-type nitride semiconductor layer nearto said active layer side than said first n-type nitride semiconductorlayer.
 13. The nitride semiconductor device according to claim 11,wherein said p-type nitride semiconductor layers include a p-typecontact layer of Al_(f)Ga_(1-f)N, (0<f<1).
 14. The nitride semiconductordevice according to claim 13, wherein said p-type contact layer has agraded composition with a high p-type impurity concentration and a smallmixed crystal ratio of Al in the substrate side.
 15. The nitridesemiconductor device according to claim 14, wherein said p-type contactlayer is composed of two layers and said two layers are anAl_(g)Ga_(1-g)N, (0<g<0.05) layer formed adjacent to said p-electrodeand an Al_(h)Ga_(1-h)N, (0<h<0.1) layer formed adjacent to the activelayer side and said Al_(g)Ga_(1-g)N, (0<g<0.05) layer has a higherp-type impurity concentration than that of said Al_(h)Ga_(1-h)N,(0<h<0.1) layer.
 16. The nitride semiconductor device according to claim11, wherein said nitride semiconductor device is a light emitting deviceand comprises a coating layer containing a phosphor substance capable ofabsorbing light from said light emitting device and emittingluminescence with wavelength difference from that of the light emittingdevice and said coating layer is formed in at least a portion of thesurface of said light emitting device.